staging: gasket: core: remove device enable and disable callbacks

Device enable/disable operations are moving from being initiated through
the gasket framework to being initiated by the gasket device driver.
The driver can perform any processing needed for these operations before
or after the calls into the framework.  Neither of these callbacks are
implemented for the only gasket driver upstream today, apex.

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