Revert "mesa: Wrap SSE4.1 code in #ifdef __SSE4_1__."

This reverts commit 8d3f739383.

In the last commit we've updated our check to determine if the actual
code is buildable, rather than if the compiler acknowledges the option.
I.e. did anyone provide -mno-sse4.1 vs is my compiler too old.

Now this code will never be attemped to be build, in both cases.

Confirmed by building mesa with
export CFLAGS='-march=native -mno-sse4.1'
./configure && make

Tested-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/main/streaming-load-memcpy.c

@@ -26,7 +26,6 @@
  *
  */
 
-#ifdef __SSE4_1__
 #include "main/macros.h"
 #include "main/streaming-load-memcpy.h"
 #include <smmintrin.h>
@@ -84,5 +83,3 @@ _mesa_streaming_load_memcpy(void *restrict dst, void *restrict src, size_t len)
       memcpy(d, s, len);
    }
 }
-
-#endif