remove moot comment

Change-Id: I0263b4ea8c60695c890f31d45198a7bd45ff3db8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220536
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

src/core/SkVM.cpp

@@ -636,4 +636,3 @@ namespace skvm {
 }
 
 // TODO: argument strides (more generally types) should come earlier, the pointers themselves later.
-// TODO: share 255 splats