*: Disable authority discovery module (#3914)

The authority discovery module enables authorities to be discoverable
and discover other authorities to improve interconnection among them. In
order to achieve this the module needs to know when the authority set
changes, thus when a session changes.

One has to register a module as a *session handler* in order for it to
be notified of changing sessions. The order and number of these *session
handlers* **MUST** correspond to the order and number of the *session
keys*.

Commit 7fc21cea added the authority discovery to the `SessionHandlers`.
Given that the authority discovery module piggybacks on the Babe session
keys the commit violated the above constraint.

This commit reverts most of 7fc21cea, leaving `core/authority-discovery`
and `srml/authority-discovery` untouched.