[clang] [clang] remove unused frontend flag -fretain-subst-template-type-parm-type-ast-nodes (PR #134177)

[Jan Voung via cfe-commits]

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

Please go ahead (need to wait for 1 to 2 weeks) and thanks again for splitting!

https://github.com/llvm/llvm-project/pull/134177