prune_worktrees() and delete_git_dir() both remove worktree
administrative entries from .git/worktrees, and their implementations
are nearly identical. The only difference is that prune_worktrees() is
also capable of removing a bogus non-worktree-related file from
.git/worktrees.
Simplify by extending delete_git_dir() to handle the little bit of
extra functionality needed by prune_worktrees(), and drop the
effectively duplicate code from the latter.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>