kernel/futex_compat.c was recently removed, but it's still in the
MAINTAINERS file. Remove it there as well.

Fixes: 04e7712f ("y2038: futex: Move compat implementation into futex.c")
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>