[mlgo] Remove the protobuf dependency

The dependency was due to the log format. This change switches to the
previously-introduced (D139370) "dependency-free" logger instead of the
protobuf-based one.

A subsequent change will clean out the unnecessary abstraction left
behind.

This change drops the logger unittest, we have sufficient test coverage
via lit tests, and a unit test would require adding, unnecesarily, a log
reader (the reader is expected to be python, for the ML side, and there
is a reader for that under Analysis/models, used for tests).

Differential Revision: https://reviews.llvm.org/D141720