[clang-tools-extra] [lld] [llvm] tools: Remove untested PluginLoader includes (PR #117644)

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

This breaks `llvm/test/Other/codegen-plugin-loading.ll` for me, which apparently was testing the `-load` command line option. I guess it should be removed? https://github.com/llvm/llvm-project/pull/182453

https://github.com/llvm/llvm-project/pull/117644