daklib/rm.py: remove unused import

The subprocess module has been unused since 400066ea
(daklib/rm.py: format date in python).

daklib/rm.py

@@ -46,7 +46,6 @@ import fcntl
 import functools
 import six
 import sqlalchemy.sql as sql
-import subprocess
 import email.utils
 from re import sub
 from collections import defaultdict