Remove WKD sync service once pacman syncs existing keys natively
The script/service/timer added in !138 (merged) updates existing keys on user systems outside the context of doing updates with pacman. This is because pacman currently only has the capability of retrieving non-existing keys (e.g. from WKD), when it encounters a signature of an unknown key, but does not retrieve an updated key for existing keys, when encountering invalid signatures.
Once pacman is able to update existing keys natively (using gpgme), we need to drop the script, service and timer again.