Revert "[Clang][Sema] Diagnose function/variable templates that shadow their...

Revert "[Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (#78274)"

This reverts commit fc025326.

This errors is disruptive to downstream projects
and should be reintroduced as a separate on-by-default
warning.

https://github.com/llvm/llvm-project/pull/78274