c++: Remove superflous assert [PR104403]

I've added the assert because start_decl diagnoses such vars for C++20 and
earlier:
  if (current_function_decl && VAR_P (decl)
      && DECL_DECLARED_CONSTEXPR_P (current_function_decl)
      && cxx_dialect < cxx23)
but as can be seen, we cam trigger the assert in older standards e.g. during
non-manifestly constant evaluation.  Rather than refining the assert that
DECL_EXPRs for such vars don't appear for C++20 and older if they are inside
of functions declared constexpr this patch just removes the assert, the
code rejects encountering those vars in constant expressions anyway.

2022-02-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104403
	* constexpr.cc (cxx_eval_constant_expression): Don't assert DECL_EXPRs
	of TREE_STATIC vars may only appear in -std=c++23.

	* g++.dg/cpp0x/lambda/lambda-104403.C: New test.