x86: remove long dead cyrix mtrr code

cyrix_arr_init was #if 0 all the way back to at least v2.6.12.

This was the only place where arr3_protected was set to anything
but zero.  Eliminate this variable.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>