Remove the specialised 'appservice' worker type

[reivilibre]

Part of #10441.

What I'm up to: Removing the synapse.app.appservice app type, letting you instead configure that with a global config option.

What the configs will look like:

Old	Transitional	New
main notify_appservices: false worker1 worker_app: synapse.app.appservice	main notify_appservices: false notify_appservices_from_worker: worker1 worker1 worker_app: synapse.app.appservice	main notify_appservices_from_worker: worker1 worker1 worker_app: synapse.app.generic_worker

[dklimpel]

Relates to

• Replace the synapse.app.appservice and synapse.app.user_dir worker types with more flexible config options #10616
• Remove the concept of worker "types" #10441