[llvm] [bazel] Revert "[bazel] llvm-config.h: Turn on LLVM_ENABLE_PLUGINS" as it fails TensorFlow Windows build (PR #143419)

[Reid Kleckner via llvm-commits]

rnk wrote:

This change broke clang-cl specifically. When you say non-MSVC, do you include clang-cl in that, or do you mean you want to enable plugins only for mingw LLVM builds?

https://github.com/llvm/llvm-project/pull/143419