Fix ApplicationTransaction "no effect" error for mentions of already-mentioned users

Summary:
Fixes T3139. See that task for discussion.

If all mentions are removed because they're already subscribed, we currently generate an empty transaction, which later gets picked up as having no effect and the user gets yelled at.

Instead, don't generate a transaction if no PHIDs remain after filtering already-subscribed PHIDs.

Test Plan: Followed plan in T3139.

Reviewers: garoevans, btrahan

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T3139

Differential Revision: https://secure.phabricator.com/D6048