hghave: remove unused "as ex" in exception block

I overlooked this when removing a debug print in another change.

Differential Revision: https://phab.mercurial-scm.org/D2637

tests/hghave.py

@@ -715,5 +715,5 b' def has_xdiff():'
         from mercurial import policy
         bdiff = policy.importmod('bdiff')
         return bdiff.xdiffblocks('', '') == [(0, 0, 0, 0)]
-    except (ImportError, AttributeError) as ex:
+    except (ImportError, AttributeError):
         return False