Revert adding .git-revision unconditionally

This reverts commit 1e534e23

.

We already should have a .git directory if it is managed via Git,
otherwise there is no way to get the Git revision if neither
.git-revision or .git is present.

But having .git-revision _and_ .git present seems very much redundant to
me.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @bennofs, @Profpatsch
Issue: #17218