Remove now unused portions of 3p mediator

[dlongley]

Some features of the 3p mediator are no longer used on any browser -- now that every browser is presumed to have partitioned storage. Any features that were previously used for an "integrated" CHAPI flow should be removed as all flows go through 1p popup windows now.

Currently, this also includes the "Remember me" feature -- which we never implemented for 1p mediators; it may not be possible to do so given user activation requirements. Either way, the 3p mediator use of it should be removed.