This check is not in all our repos that use git-version-gen. Indeed it
seems to be a leftover of openbsc where I think it wanted to ensure
being called in the openbsc subfolder or something? libosmocore e.g.
doesn't have it.
In any case .git being a directory is not always true (if using git
worktree) so remove this check.
This is the first step in creating this repository from the legacy openbsc.git.
Like all other Osmocom repositories, keep the autoconf and automake files in
the repository root. openbsc.git has been the sole exception, which ends now.