python: drop unnecessary conn assignment

Since 554d82a200, conn is unused. Let's
drop it - but keep the signature of the constructor for backward
compatibility.

python/libvirt-override.py

@@ -18,15 +18,6 @@ import types
 class libvirtError(Exception):
     def __init__(self, defmsg, conn=None, dom=None, net=None, pool=None, vol=None):
 
-        if dom is not None:
-            conn = dom._conn
-        elif net is not None:
-            conn = net._conn
-        elif pool is not None:
-            conn = pool._conn
-        elif vol is not None:
-            conn = vol._conn
-
         # Never call virConnGetLastError().
         # virGetLastError() is now thread local
         err = virGetLastError()