staging: gasket: remove gasket_page_table_num_extended_entries()

It is exported, yet no one calls it 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>