cmd/dist: remove GOTESTONLY environment variable

cmd/dist: remove GOTESTONLY environment variable

This hook was added for the Go build system (x/build) to be able to set
the run flag value, but it's no longer used anywhere. Remove it for now.

Updates #46054.

Change-Id: I64e7d68d2b270303f3bd54f73079600f209e350a
Reviewed-on: https://go-review.googlesource.com/c/go/+/455519
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>