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