[clang] [clang] Remove the deprecated flag `-frelaxed-template-template-args`. (PR #111894)

[Erich Keane via cfe-commits]

erichkeane wrote:

I THINK I'd still prefer to keep this flag for 1 more release. Our mitigations /P3310 haven't been in a release cycle yet, so I'd prefer we wait until after the 20 release to flip this switch.

https://github.com/llvm/llvm-project/pull/111894