Don't use the Sk4f gradient impl without SIMD

Also remove the SK_SUPPORT_LEGACY_LINEAR_GRADIENT_TABLE guard since it is no
longer used in Chromium.

BUG=chromium:563492
R=reed@google.com,mtklein@google.com
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1489233005
4 files changed