[clang] NFCI: remove obsolete workaround for template default arguments

This removes a workaround for template template arguments pointing to
older template declarations, which don't have the most recent default
argument definition.

The removed workaround was introduced in 1abacfcb,
but https://github.com/llvm/llvm-project/pull/75569 introduced a
better fix which is more comprehensive and doesn't require rebuilding
TemplateNames.