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 <dmitshur@golang.org>

Author: Bryan C. Mills

src/cmd/go/testdata/script/mod_convert_dep.txt

@@ -20,7 +20,6 @@ cd $WORK/gopkgdir/x
 ! go list .
 stderr 'cannot find main module'
 ! stderr 'Gopkg.lock'
-! stderr 'go mod init'
 
 -- $WORK/test/Gopkg.lock --
 -- $WORK/test/x/x.go --