tests: remove undefined (empty) $opt from test-extdiff.t

It's been undefined for a long time (at least since .t unification).

Differential Revision: https://phab.mercurial-scm.org/D9621

tests/test-extdiff.t

@@ -11,7 +11,7 b''
 
 Should diff cloned directories:
 
-  $ hg extdiff -o -r $opt
+  $ hg extdiff -o -r
   Only in a: a
   Only in a: b
   [1]