Revert "Check for existence of refs upon initial fetch"

This reverts commit 56548058.

We are reverting this change for 2 reasons:

1) It introduced a bug for users using sync -c with a reference mirror.
2) The fetch specs have recently changed to cause git to properly fail
when we request a non-existent branch of a manifest, removing the need
for this change.

Change-Id: I0f63da9bfb40cf5ffafb7979f1b8c929a738fc7b