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