Remove SkShaders::Lerp

It's entirely unused, and trivial for clients to create with SkSL.

Change-Id: I197986232d3706f5af3a197f0fb8e744e1009e5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419796
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Reed <reed@google.com>
11 files changed