x86: delete the arch-specific dma-mapping headers.

all the code that is left is ready to be merged as-is
in dma-mapping.h.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>