Remove ML inlining model artifacts.

They are not conducive to being stored in git. Instead, we autogenerate
mock model artifacts for use in tests. Production models can be
specified with the cmake flag LLVM_INLINER_MODEL_PATH.

LLVM_INLINER_MODEL_PATH has two sentinel values:
 - download, which will download the most recent compatible model.
 - autogenerate, which will autogenerate a "fake" model for testing the
 model uptake infrastructure.

Differential Revision: https://reviews.llvm.org/D104251