[Polly] Remove Polly-ACC.

Polly-ACC is unmaintained and since it has never been ported to the NPM pipeline, since D136621 it is not even accessible anymore without manually specifying the passes on the `opt` command line.

Since there is no plan to put it to a maintainable state, remove it from Polly.

Reviewed By: grosser

Differential Revision: https://reviews.llvm.org/D142580