x86/asm: Remove unused __constant_c_x_memset() macro and inlines

Nothing refers to the __constant_c_x_memset() macro anymore. Remove
it and the two referenced static inline functions.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190111084931.24601-2-linux@rasmusvillemoes.dk