remove unused exception binding in sandbox.py

pyflakes 2.0 is more strict and found that 'e' is never used.

References: https://build.opensuse.org/request/show/629206 (comment
section)

PR: https://gitlab.com/apparmor/apparmor/merge_requests/178
(cherry picked from commit 51482c33f5)
Signed-off-by: John Johansen <john.johansen@canonical.com>

utils/apparmor/sandbox.py

@@ -718,7 +718,7 @@ def run_xsandbox(command, opt):
     # aa-exec
     try:
         rc, report = aa_exec(command, opt, x.new_environ, required_rules)
-    except Exception as e:
+    except Exception:
         x.cleanup()
         raise
     x.cleanup()