x86: unexport kmap_atomic_to_page

This patch removes the no longer used export of kmap_atomic_to_page.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>