staging: android: ion: Remove unused declaration ion_buffer_fault_user_mappings

ion_buffer_fault_user_mappings's definition has been removed and not be
used anymore, just remove its useless declaration.

Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>