rm: remove leading and trailing whitespace from removal reason

It looks like some removal records had trailing whitespace which
confuses users of the deb822 file.

Reference: https://bugs.debian.org/960504

daklib/rm.py

@@ -366,6 +366,7 @@ def remove(session, reason, suites, removals,
 
     if not reason:
         raise ValueError("Empty removal reason not permitted")
+    reason = reason.strip()
 
     if not removals:
         raise ValueError("Nothing to remove!?")