util: drop util.Abort in favor of error.Abort (API)

IIRC, error.Abort exists since Mercurial 1.2, so it should be pretty easy
for extensions authors to update their code.

.. api::

   The util.Abort alias has been removed. Use error.Abort.

mercurial/util.py

@@ -1586,8 +1586,6 def increasingchunks(source, min=1024, m
     if buf:
         yield ''.join(buf)
 
-Abort = error.Abort
-
 def always(fn):
     return True