Remove EffectiveModelContext{Listener,Provider}

Remove EffectiveModelContext{Listener,Provider}

These are useless things that introduce verbosity.

At the end of the day, EffectiveModelContextProvider is someone who has

  EffectiveModelContext modelContext()

method -- which is but an idiom.

If we need to throw in a source of ISEs and callbacks, we are modeling
something much different -- better expressed explicitly where needed.

Change-Id: I6b1c11b01a0656a6a373e0d9b940607ee7a20aa0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>