Remove overly-promiscuous SkNx syntax sugar.

I haven't figured out a pithy way to have these apply to only classes
originating from SkNx, so let's just remove them.  There aren't too
many use cases, and it's not really any less readable without them.

Semantically, this is a no-op.

BUG=skia:

Review URL: https://codereview.chromium.org/1167153002
diff --git a/bench/PMFloatBench.cpp b/bench/PMFloatBench.cpp
index 14ccb33..37542e3 100644
--- a/bench/PMFloatBench.cpp
+++ b/bench/PMFloatBench.cpp
@@ -121,10 +121,10 @@
                 fDevice[i+1] = SkPMFloat(b).trunc();
                 fDevice[i+2] = SkPMFloat(c).trunc();
                 fDevice[i+3] = SkPMFloat(d).trunc();
-                a += dcdx4;
-                b += dcdx4;
-                c += dcdx4;
-                d += dcdx4;
+                a = a + dcdx4;
+                b = b + dcdx4;
+                c = c + dcdx4;
+                d = d + dcdx4;
             }
         }
     }