refactor: Remove unused CC_TOOLCHAIN definition

[katre]

Fixes #2979.

The definition appears unused and helps advance the goal of entirely removing current_cc_toolchain: see bazelbuild/bazel#26282.

[katre]

@rickeylev Do you know who should review this?

[rickeylev]

I'd be the most appropriate.

Hm, I swear when I was grepping around I saw _cc_toolchain somewhere else, too. In any case, this still valid cleanup. LGTM.

Regardless, usage of the cc toolchain should be unused. The Bazel py rules don't build a C++ artifact like the Google py rules do.