ARM: 9211/1: domain: drop modify_domain()

This function/macro isn't used anywhere in the kernel.
The only user was set_fs() and was deleted in the set_fs()
removal patch set.

Fixes: 8ac6f5d7 ("ARM: 9113/1: uaccess: remove set_fs() implementation")
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>