asm-generic: page.h: Remove useless get_user_page and free_user_page

They are not symmetric with each other, neither are used in real world
(can not be found by grep command in source code root directory), so
remove them.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>