Cleanup stale manifest migration code from manifest.py

Prior to open-sourcing repo we had manifests in two different
layouts; one where the manifest was a straight-up git clone, and
one where the manifest was our bare repository with symlink work
tree format (identical to what our projects use).  Only the latter
form is created or used by repo at this point, so the transition
code to handle the straight-up git clone is not necessary.

Signed-off-by: Shawn O. Pearce <sop@google.com>

manifest.py

@@ -45,16 +45,9 @@ class Manifest(object):
       gitdir   = os.path.join(repodir, 'repo/.git'),
       worktree = os.path.join(repodir, 'repo'))
 
-    wt     = os.path.join(repodir, 'manifests')
-    gd_new = os.path.join(repodir, 'manifests.git')
-    gd_old = os.path.join(wt, '.git')
-    if os.path.exists(gd_new) or not os.path.exists(gd_old):
-      gd = gd_new
-    else:
-      gd = gd_old
     self.manifestProject = MetaProject(self, 'manifests',
-      gitdir   = gd,
-      worktree = wt)
+      gitdir   = os.path.join(repodir, 'manifests.git'),
+      worktree = os.path.join(repodir, 'manifests'))
 
     self._Unload()