clang: remove PROVIDES:append settings

[ChenQi1989]

With these settings, we'll have multiple providers of llvm. This means that with pristine poky master + meta-clang master, we'll have some output like below:

NOTE: Multiple providers are available for llvm-native (llvm-native, clang-native)
Consider defining a PREFERRED_PROVIDER entry to match llvm-native

And when running 'bitbake world', we'll get error messages like below:

ERROR: Multiple .bb files are due to be built which each provide llvm-native:
virtual:native:/PATH/TO/poky/meta/recipes-devtools/llvm/llvm_19.1.4.bb
virtual:native:/PATH/TO/Yocto/poky/meta-clang/recipes-devtools/clang/clang_git.bb

As the README.md has already specified how to use this layer, let's remove such settings before sorting things out.

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[kraj]

lgtm