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.
Change-Id: Ic14561f3b041bb94d1b60e477b18e37077ce4c32