Revert of Implement four more xfermodes with Sk4px.

src/opts/SkXfermode_opts_arm_neon.cpp

 // Copyright 2013 unknown
 
 #include "SkXfermode.h"
 #include "SkXfermode_proccoeff.h"
 #include "SkColorPriv.h"
 
 #include <arm_neon.h>
 #include "SkColor_opts_neon.h"
 #include "SkXfermode_opts_arm_neon.h"
 #include "Sk4pxXfermode.h"
@@ skipping 998 matching lines @@
 SK_COMPILE_ASSERT(
     SK_ARRAY_COUNT(gNEONXfermodeProcs1) == SkXfermode::kLastMode + 1,
     mode1_count_arm
 );
 
 SkProcCoeffXfermode* SkPlatformXfermodeFactory_impl_neon(const ProcCoeff& rec,
                                                          SkXfermode::Mode mode) {
     if (auto xfermode = SkCreate4pxXfermode(rec, mode)) {
         return xfermode;
     }
-    // TODO: Sk4pxXfermode now covers every mode found in this file.  Delete them all!
     if (auto proc = gNEONXfermodeProcs[mode]) {
         return SkNEW_ARGS(SkNEONProcCoeffXfermode, (rec, mode, (void*)proc));
     }
     return NULL;
 }
 
 SkXfermodeProc SkPlatformXfermodeProcFactory_impl_neon(SkXfermode::Mode mode) {
     return gNEONXfermodeProcs1[mode];
 }