Remove AltiVec code

The content of src/archutils/Darwin/VectorHelper.cpp is very old, these are AltiVec and SSE2 instructions. This is only actually used in RageSoundMixBuffer.cpp. This level of micro-optimization is not needed since platforms this old are not supported by ITGm.

src/RageSoundMixBuffer.cpp

@@ -5,13 +5,6 @@
 #include <cmath>
 #include <cstdint>
 
-#if defined(MACOSX)
-#include "archutils/Darwin/VectorHelper.h"
-#ifdef USE_VEC
-static bool g_bVector = Vector::CheckForVector();
-#endif
-#endif
-
 RageSoundMixBuffer::RageSoundMixBuffer()
 {
 	m_iBufSize = m_iBufUsed = 0;
@@ -52,21 +45,12 @@ void RageSoundMixBuffer::write( const float *pBuf, unsigned iSize, int iSourceSt
 	if( iSize == 0 )
 		return;
 
-	/* iSize = 3, iDestStride = 2 uses 4 frames.  Don't allocate the stride of the
-	 * last sample. */
+	// iSize = 3, iDestStride = 2 uses 4 frames.  Don't allocate the stride of the last sample.
 	Extend( iSize * iDestStride - (iDestStride-1) );
 
-	/* Scale volume and add. */
+	// Scale volume and add.
 	float *pDestBuf = m_pMixbuf+m_iOffset;
 
-#ifdef USE_VEC
-	if( g_bVector && iSourceStride == 1 && iDestStride == 1 )
-	{
-		Vector::FastSoundWrite( pDestBuf, pBuf, iSize );
-		return;
-	}
-#endif
-
 	while( iSize )
 	{
 		*pDestBuf += *pBuf;