remove skvx::{rsqrt,rcp}

These don't return reliable portable results, so I don't want to promote
them as good ideas to use.  You can get at least 5 different results
from these across the four main architectures we support, and they've
been the root cause of bugs uncovered only in production on undertested
platforms.

Luckily, unused outside of tests.

Change-Id: I532731fe4cddf127253341e5ace8d9c5c9ebb0f1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326108
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed