[libc++] Remove libc++ Debian builders

In accordance with the announcement on libcxx-dev and llvm-dev [1],
we are removing the libc++ specific buildbots in favor of running
those configurations in the pre-commit CI on Buildkite. This commit
removes the Debian-based libc++ build bots.

[1]: http://lists.llvm.org/pipermail/libcxx-dev/2020-November/001040.html

Differential Revision: https://reviews.llvm.org/D92862