staging: gasket: core: delete device add and remove callbacks

Gasket device drivers are now in charge of orchestrating the device add
and removal sequences, so the callbacks from the framework to the device
drivers for these events are no longer needed.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>