filter-repo (python3): revert "workaround python<2.7.9 exec bug"

Commit ca32c5d9afe2 ("filter-repo: workaround python<2.7.9 exec bug",
2019-04-30) put in a workaround for python versions prior to 2.7.9, but
which was incompatible with python3.  Revert it as one step towards
migrating to python3.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -2619,8 +2619,8 @@ class RepoFilter(object):
 
   def _handle_arg_callbacks(self):
     def make_callback(argname, str):
-      exec 'def callback({}):\n'.format(argname)+\
-           '  '+'\n  '.join(str.splitlines()) in globals()
+      exec('def callback({}):\n'.format(argname)+
+           '  '+'\n  '.join(str.splitlines()), globals())
       return callback #namespace['callback']
     def handle(type):
       callback_field = '_{}_callback'.format(type)