[staging] llvmPackages_*.clang: stop passing LLVM_CONFIG_PATH unnecessarily

[sternenseemann]

Starting with LLVM 8, clang does no longer use llvm-config to detect the
LLVM installation: llvm/llvm-project@e4faa5c7986b7
Consequently, there is no point passing LLVM_CONFIG_PATH (in fact the
variable is unused currently).

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[sternenseemann]

@ofborg build llvmPackages_8.clang-unwrapped

[sternenseemann]

@ofborg build llvmPackages_13.clang-unwrapped

[sternenseemann]

@ofborg build llvmPackages_11.clang-unwrapped

[Mindavi]

Isn't this needed for mesa cross-compilation?

Edit: oh, it's only for clang. Then I guess it isn't needed.

[Ericson2314]

Horry to one less annoying config tool!

[github-actions]

Backport failed for staging-21.11, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin staging-21.11
git worktree add -d .worktree/backport-153340-to-staging-21.11 origin/staging-21.11
cd .worktree/backport-153340-to-staging-21.11
git checkout -b backport-153340-to-staging-21.11
ancref=$(git merge-base c6f7ec4c2a6b2910f53d083fd2a8d191d72758ac c58517aeed3ee840a0d92455e4f4231f2396d73f)
git cherry-pick -x $ancref..c58517aeed3ee840a0d92455e4f4231f2396d73f