cmd/go: remove experiment checks for compile -c

cmd/go: remove experiment checks for compile -c

There's a comment that we should test that compile -c is compatible with
the fieldtrack and preemptibleloops experiments and then remove the
check disabling -c when those experiments are enabled.

I tested this and the tests pass with fieldtrack (with the exception of one go command test that makes the assumption that fieldtrack is off), and the preemptibleloops experiment is already broken without this experiment.

Also remove the check for the value of the GO19CONCURRENTCOMPILATION
environment variable. The compiler concurrency can be limited by setting
GOMAXPROCS.

Change-Id: I0c02745de463ea572673648061185cd76a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/724680
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>