Remove deprecated model manager.

[craiga]

Problem

Fixes #364 by not using SoftDeletableManager as the default manager for SoftDeletableModel.

Solution

Explain the solution that has been implemented, and what has been changed.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[codecov]

Codecov Report

Merging #480 (afe4ac9) into master (9d0dfc3) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   94.57%   94.52%   -0.06%     
==========================================
  Files           6        6              
  Lines         756      749       -7     
==========================================
- Hits          715      708       -7     
  Misses         41       41              

Impacted Files	Coverage Δ
model_utils/managers.py	98.04% <ø> (-0.06%)	⬇️
model_utils/models.py	98.91% <100.00%> (-0.02%)	⬇️

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[tadamcz]

Would be great to get this into the next major version!

[foarsitter]

@craiga I know it has been a while but I think this PR is a good candidate for the 5.0 release. Are you willing to and do you have time to rebase?