[llvm] [libcxx] [libc++][modules] Removes module testing. (PR #76083)

[Mark de Wever via llvm-commits]

mordante wrote:

> Obviously I think that much simplification is nice, but shouldn't we wait until we have the replacement before we merge this removal?

I have a replacement and I want to stack that on top of this patch once the CI is green. The intention is to disable module testing for a few minutes. However I want to avoid mixing the patches to give a better view of which change does what.

https://github.com/llvm/llvm-project/pull/76083