Remove experimental C API

We have no internal clients using this, and it is not being
maintained/updated. This would be one more thing to port
to Bazel and maintain backwards compatibility GN rules for,
so if we can delete it, that would make those tasks easier.

Change-Id: Ia77a77903f60acf3e611f1750c9225b8933de176
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/567637
Reviewed-by: Brian Salomon <bsalomon@google.com>
33 files changed