cmd/go: 'go help get' describes nonexistent 'go help gopath-get'

[findleyr]

At the bottom of go help get, a reference is made to go help gopath-get, yet this help no longer exists (probably as of Go 1.22, when go get stopped working for GOPATH mode).

We should remove that reference, and update surrounding discussion of GOPATh mode.

[gabyhelp]

Related Issues

• cmd/go: `go help get` should show module-get not GOPATH get #37103 (closed)
• cmd/go: fix typo in output of go help importpath #29605 (closed)
• cmd/go: fix typo in output of "go help get" #28401 (closed)

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