bdiff-torture: fix pyflakes warning reporting undefined name 'inst'

Looks like I got a latest version of pyflakes somehow or it's running on py3 and
it spotted this.

Differential Revision: https://phab.mercurial-scm.org/D6757

contrib/bdiff-torture.py

@@ -53,7 +53,6 b' def testwrap(a, b):'
         test1(a, b)
         return
     except Exception as inst:
-        pass
     print("exception:", inst)
     reducetest(a, b)