gdb: add interp::on_inferior_disappeared method

Same idea as previous patches, but for inferior_disappeared.

For symmetry with on_inferior_appeared, I named this one
on_inferior_disappeared, despite the observer being called
inferior_exit.  This is called when detaching an inferior, so I think
that calling it "disappeared" is a bit less misleading (the observer
should probably be renamed later).

Change-Id: I372101586bc9454997953c1e540a2a6685f53ef6