Remove dual hack

[devmotion]

This PR removes a "Hack for conversions", as it's called in the comment.

The problem is that

• it is a very severe form of type piracy that completely changes the behaviour of ForwardDiff.Duals when Tracker is loaded
• it causes downstream issues (see e.g. ReverseDiffVJP errors due to method ambiguity for DAE adjoint SciML/SciMLSensitivity.jl#724)
• the underlying issue should not be addressed here but upstream (see Add FloatNN(::Dual) JuliaDiff/ForwardDiff.jl#538)
• the definition messes up the dual number tags since it just sets them to Nothing (cf. with the definition in the ForwardDiff PR), and hence the following example still does not work but only fails with a different error message:

julia> using ForwardDiff

julia> ForwardDiff.derivative(x -> Float64(x), 5)
ERROR: MethodError: no method matching Float64(::ForwardDiff.Dual{ForwardDiff.Tag{var"#3#4", Int64}, Int64, 1})
Closest candidates are:
  (::Type{T})(::Real, ::RoundingMode) where T<:AbstractFloat at rounding.jl:200
  (::Type{T})(::T) where T<:Number at boot.jl:772
  (::Type{T})(::AbstractChar) where T<:Union{AbstractChar, Number} at char.jl:50
  ...
Stacktrace:
 [1] (::var"#3#4")(x::ForwardDiff.Dual{ForwardDiff.Tag{var"#3#4", Int64}, Int64, 1})
   @ Main ./REPL[3]:1
 [2] derivative(f::var"#3#4", x::Int64)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/pDtsf/src/derivative.jl:14
 [3] top-level scope
   @ REPL[3]:1

julia> using Tracker

julia> ForwardDiff.derivative(x -> Float64(x), 5)
ERROR: Cannot determine ordering of Dual tags Nothing and ForwardDiff.Tag{var"#5#6", Int64}
Stacktrace:
 [1] ≺(a::Type, b::Type)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/pDtsf/src/dual.jl:54
 [2] partials
   @ ~/.julia/packages/ForwardDiff/pDtsf/src/dual.jl:108 [inlined]
 [3] extract_derivative
   @ ~/.julia/packages/ForwardDiff/pDtsf/src/derivative.jl:84 [inlined]
 [4] derivative(f::var"#5#6", x::Int64)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/pDtsf/src/derivative.jl:14
 [5] top-level scope
   @ REPL[5]:1

[coveralls]

Pull Request Test Coverage Report for Build 3010223394

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 72.353%

---

Totals
Change from base Build 2905161731:	0.2%
Covered Lines:	492
Relevant Lines:	680

---

💛 - Coveralls

[codecov-commenter]

Codecov Report

Merging #134 (f3130be) into master (187b472) will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   72.14%   72.35%   +0.21%     
==========================================
  Files           9        9              
  Lines         682      680       -2     
==========================================
  Hits          492      492              
+ Misses        190      188       -2     

Impacted Files	Coverage Δ
src/lib/real.jl	67.14% <ø> (+0.47%)	⬆️
src/back.jl	90.69% <0.00%> (+0.77%)	⬆️

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[ChrisRackauckas]

Yes, the fact that this messes up the tags means it's not even useful, and is dangerous. I'm merging and tagging this. The nightly failure looks unrelated.

[coveralls]

Pull Request Test Coverage Report for Build 3010223394

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 72.353%

---

Totals
Change from base Build 2905161731:	0.2%
Covered Lines:	492
Relevant Lines:	680

---

💛 - Coveralls