Remove unused CONST_CS flag

[DanielEScherzer]

No description provided.

[DanielEScherzer]

Pretty confident that the failure is unrelated:

========DIFF========
001+ [0454] Expecting string/2024-12-06 21:07:39, got string/2024-12-06 21:07:38 resp. string/2024-12-06 21:07:38. [0] 
     OK
========DONE========
FAIL mysqli_fetch_all() data types variation [ext/mysqli/tests/fetch/mysqli_fetch_all_data_types_variation.phpt] 

[cmb69]

Generally fine, but might be better to wait for the next major version of PHP.

[kocsismate]

Yes, this change is an ABI break which 3rd party extension have to account for. Since gen_stub.php was recently got rid of using this flag, there are possibly still quite some usages. In order to avoid unnecessarily nagging maintainers, I'd also vote for delaying the removal until PHP 9.0.