Revert "Add --prune option to fetch when syncing a mirror repo"

For some users it is not desirable to remove refs that don't exist
on the remote server when syncing a mirror repo.

This reverts commit b4d43b9f664d6472b6c1e91c98f951037d00cea5.

Change-Id: Ie849b66682138ef88da6cd1a5fbb27e993197dd7

project.py

@@ -1881,8 +1881,6 @@ class Project(object):
       cmd.append('--quiet')
     if not self.worktree:
       cmd.append('--update-head-ok')
-    if self.manifest.IsMirror:
-      cmd.append('--prune')
     cmd.append(name)
 
     # If using depth then we should not get all the tags since they may