Revert of Implement four more xfermodes with Sk4px.

src/opts/SkXfermode_opts_arm_neon.cpp

Index: src/opts/SkXfermode_opts_arm_neon.cpp
diff --git a/src/opts/SkXfermode_opts_arm_neon.cpp b/src/opts/SkXfermode_opts_arm_neon.cpp
index 205a00b4ce2cbc0beba2846de697dbcef99e6b56..1759429c574364b2fb0b78a933ff7799f026ea1a 100644
--- a/src/opts/SkXfermode_opts_arm_neon.cpp
+++ b/src/opts/SkXfermode_opts_arm_neon.cpp
@@ -1016,7 +1016,6 @@
     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));
     }