remove old-style 4-float xfemrodes
remove old-style 4-float xfemrodes

These routines are used in legacy 8888, but the default implementation
that builds a little raster pipeline is actually faster for these.

I'm also looking to get rid of some of the other routines in this
file, but these three look like the easy place to start.

Guarded by SK_LEGACY_4F_XFERMODES.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3388d360652ad93282bd842277162f680fb67580
Reviewed-on: https://skia-review.googlesource.com/c/160880
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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