Intrinsics: drop custom _mm_shuffle_epi8()

We don't actually use it, because of reasons.

Source/Core/Common/Intrinsics.h

@@ -12,26 +12,6 @@
 #include <x86intrin.h>
 #endif
 
-#ifndef __GNUC_PREREQ
-#define __GNUC_PREREQ(maj, min) 0
-#endif
-
-#if defined __GNUC__ && !__GNUC_PREREQ(4, 9) && !defined __SSSE3__
-// GCC <= 4.8 only enables intrinsics based on the command-line.
-// GCC >= 4.9 always declares all intrinsics.
-// We only want to require SSE2 and thus compile with -msse2,
-// so define the one post-SSE2 intrinsic that we dispatch at runtime.
-static __inline __m128i __attribute__((__gnu_inline__, __always_inline__, __artificial__))
-_mm_shuffle_epi8(__m128i a, __m128i mask)
-{
-	__m128i result;
-	__asm__("pshufb %1, %0"
-		: "=x" (result)
-		: "xm" (mask), "0" (a));
-	return result;
-}
-#endif
-
 #if defined _M_GENERIC
 #  define _M_SSE 0
 #elif _MSC_VER || __INTEL_COMPILER