Revert "[ARM] pxa: introduce cpu_is_pxa26x()"

This reverts commit da1a3dc0eb.

The originally proposed way in the above commit is incorrect. And
there is no easy way to distinguish between pxa25x and pxa26x at
run-time.

Signed-off-by: Eric Miao <eric.miao@marvell.com>

arch/arm/mach-pxa/include/mach/hardware.h

@@ -204,8 +204,6 @@
 		__cpu_is_pxa25x(read_cpuid_id());	\
 	})
 
-extern int cpu_is_pxa26x(void);
-
 #define cpu_is_pxa27x()					\
 	({						\
 		__cpu_is_pxa27x(read_cpuid_id());	\