we don't need two ways to disable lowp

SK_DISABLE_LOWP_RASTER_PIPELINE is a little nicer than the old define,
as when enabled we won't even build the lowp stages.

Change-Id: I51d6ca308d8efbb1fc5693d8091a1e6c6d243bf2
Reviewed-on: https://skia-review.googlesource.com/c/164628
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed