cmd/go: remove a bogus assertion in mod_convert_dep
cmd/go: remove a bogus assertion in mod_convert_dep

The removed line assumed that the script's WORK directory is not a
child of any directory containing version-control metadata.
While that assumption does hold in most cases, it does not hold when,
for example, $TMPDIR is $HOME/tmp and $HOME/.git/config exists.

A similar situation may or may not arise when using
golang.org/x/build/cmd/release. Either way, the assertion is incorrect
and was interfering with local testing for #39385.

Updates #39385
Fixes #39431

Change-Id: I67813d7ce455aa9b56a6eace6eddebf48d0f7fa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/236818
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

akutz/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,akutz/go,nawawi/go,akutz/go,golang/go,golang/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go