contrib: remove "git-new-workdir"

contrib: remove "git-new-workdir"

The "git-new-workdir" command has been introduced to make it possible to
have a separate working directory in a different place. The command thus
predates git-worktree(1), which is what people use nowadays to create
any such working directory. As such, the script doesn't really have much
of a reason to exist nowadays anymore.

It also doesn't seem like the script is still in use: the last time it
has received an update was in e32afab7b03 (git-new-workdir: don't fail
if the target directory is empty, 2014-11-26), more than a decade ago.
Remove it as well as the tests that depend on it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>