Summary:
Fix CoreAndCallbackReference move-ctor so it doesn't call this->detach() (my bad; see D7723078).

Also (for clarity/pedantry/obviousness), explicitly delete the class' move-assignment op.

Reviewed By: yfeldblum

Differential Revision: D7848370

fbshipit-source-id: bc051120b3df0557ce9b01cbad8c00a57b8780d8