Revert "cmd/go: fix the default build output name for versioned binaries"

This reverts CL 140863 (commit bf94fc3a).

Reason for revert: There was a potential problem spotted in the original
fix, which resulted in it being rolled back from release-branch.go1.12
and not included in Go 1.12.1 release. We intend to improve the fix and
include it in Go 1.12.2 instead. To make the fix easier to backport,
revert this change before re-applying the improved fix (next commit).

Change-Id: If6c785f58482d2531b5927c5ea7002f548c21c7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/168402


Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>