Fix invalid logic in PhabricatorEmailPreferencesSettingsPanel::processRequest()
Summary:
The expression `$all_tags[$tag]['count']` does not do anything. Reading the surrounding code, intention is to display a "Common" section on `/settings/panel/emailpreferences/` listing MailTags defined by at least two different applications. (This is currently not the case anyway as Phorge prefixes all MailTags with their corresponding application but might be a future use case.)
This change fixes the logic accordingly.
Closes T15874
Test Plan: Apply the patch; replace the value of a random `const MAILTAG_*` line in the codebase with the value of another random `const MAILTAG_*` line in the codebase to make it "common", now see that `/settings/panel/emailpreferences/` displays this MailTag in a "Common" section on top of the options.
Reviewers: O1 Blessed Committers, speck
Reviewed By: O1 Blessed Committers, speck
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15874
Differential Revision: https://we.phorge.it/D25727