Remove non-protobuf path.

It is expensive to keep the non-protobuf path around and it hasn't
been used for an entire release without anyone noticing, so remove it.

Create new end-to-end unit tests that cover tests of the non-proto
code paths that are being deleted.

Bug: 197981919

Test: Unit tests pass.
Change-Id: Ia1c45572300bd63e5f196ad61e5e5386830c8ece