R1:ff7be2cf1444

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.