Clean up more SkXfermode.cpp dead code.

These handwritten xfermodes for Clear, Src, DstIn, and DstOut are actually dead
code: they're all covered by Sk4pxXfermode, which we'd already have returned.

Tidies up the xfermode creation logic to make this clearer.

This cuts 20-40K off SkXfermode.o, depending on the platform.

BUG=skia:

Review URL: https://codereview.chromium.org/1249773004
1 file changed