Remove webcals support

The webcals is not officially registered (it's only assumed to be webcal
over https), and it seems [1] some clients
don't support it.

Closes #2416 and #3050 and also related to #2197

[1] https://github.com/nextcloud/calendar/issues/2416



Signed-off-by: Thomas Citharel <tcit@tcit.fr>