libstdc++: Remove dg-options "-std=c++20" from <span> and <cuchar> tests

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

There doesn't seem to be any reason they need to use strict -std=c++20
mode, so don't add the { dg-add-options strict_std } directive, just let
them use strict or non-strict modes.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc:
	Remove dg-options.
	* testsuite/23_containers/span/contiguous_range_neg.cc:
	Likewise.
	* testsuite/23_containers/span/everything.cc: Likewise.