[clang][test] Make check pattern shorter

A check pattern in clang/test/SemaCXX/template-64605.cpp contains template
specialization kind (the text "implicit_instantiation"). It does not need to
be checked and can be safely removed.

Presence of this text in the check pattern prevents from backporting some
commits to the release branch: https://github.com/llvm/llvm-project/issues/64605.
It has only recently been printed and the relevant commit is not present in
the release/17.x branch.

(cherry picked from commit 8859c644)