cmd/go: remove TestScript/version_buildvcs_git_gpg

This was a regression test added for a 'git' command line
used for build stamping. Unfortunately, 'gpg' has proved to
be extremely fragile:

* In recent versions, it appears to always require 'gpg-agent' to be
  installed for anything involving secret keys, but for some reason is
  not normally marked as requiring gpg-agent in Debian's package
  manager.

* It tries to create a Unix domain socket in a subdirectory of $TMPDIR
  without checking the path length, which fails when $TMPDIR is too
  long to fit in the 'sun_path' field of a sockaddr_un struct (which
  typically tops out somewhere between 92 and 108 bytes).

We could theoretically address those by artificially reducing the
script's TMPDIR length and checking for gpg-agent in addition to gpg,
but arguably those should both be fixed upstream instead. On balance,
the incremental value that this test provides does not seem worth the
complexity of dealing with such a fragile third-party tool.

Updates #50675.
Updates #48802.
Fixes #57034.

Change-Id: Ia3288c2f84f8db86ddfa139b4d1c0112d67079ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/454502
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: Bryan C. Mills