[clang] [clang-tools-extra] [llvm] [clang-tidy] Remove cert-dcl21-cpp check (PR #80181)

[via llvm-commits]

https://github.com/EugeneZelenko approved this pull request.

Looks OK for me, but will be good idea if @PiotrZSL will also take a look.

https://github.com/llvm/llvm-project/pull/80181