project/sync: move DeleteProject helper to Project

Since deleting a source checkout involves a good bit of internal
knowledge of .repo/, move the DeleteProject helper out of the sync
code and into the Project class itself.  This allows us to add git
worktree support to it so we can unlock/unlink project checkouts.

Change-Id: If9af8bd4a9c7e29743827d8166bc3db81547ca50
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256072
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>