Remove many notification backends

This removes the notifyby{execute,logfile,taskbar,texttospeech} backends.

Doing these things client-side is conceptually wrong. If at all they would need to be implemented
in the notification server to provide consistent ability regardless of whether the notification originates
from KNotifications or any other means (libnotify, raw DBus, notification portal etc).

The functionality is also very niche

This only leaves the relevant platform backends (FDO, portal, windows, mac, android) as well as the sound backends.

In an ideal world the sound backends would go too, but too many apps rely on them

https://phabricator.kde.org/T11875