Remove unused TopDownMutatorContext methods

Remove methods on TopDownMutatorContext that are no longer used.
This deletes everything unique to BaseMutatorContext, so delete that
too and replace it with BaseModuleContext.

Bug: 367784740
Test: builds
Flag: EXEMPT refactor
Change-Id: Id37c7b7e3d46d75afaf7752862b08f766217f08b