x86: kill 8253pit.h

Useless header file with 32 bit and 64 bit variants. Move the
single useful line to the place where it is used.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>