staging: gasket: remove gasket_interrupt_get_msix_entries()

No one calls it, it is claimed to be "legacy", whatever that means, so
just remove the dead code.

Cc: Rob Springer <rspringer@google.com>
Cc: John Joseph <jnjoseph@google.com>
Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>