Removing Neko support/FLX_NO_HEALTH Define in Flixel 7.0.0+

Neko is in short, an dead brother of Hashlink (which has superseded Neko) and it's pretty outdated lately, it's last updated being in mid 2024 (when it was deprecated, it's not long ago but still, Neko is discontinued and it even caused 4.2.5 to fail to compile too with newer versions). And as such, since Flixel 6.0.0 has removed a bunch of deprecated stuff and given how Hashlink is more better then Neko, I'd figured we could finally deprecate then remove Neko in upcoming Flixel 6.0.0 patches, including the FLX_NO_HEALTH define that I had made/suggested since it was for backwards compatibility with older flixel projects and seemed no longer necessary since people could extend FlxBasic/FlxObject & add it themselves, I think it would be wise to remove it soon in an upcoming Flixel 6.0.0 patch since it's deprecated and soon to be removed much like how Neko should be

The issue targets Neko support and the FLX_NO_HEALTH define, but names no files, tests, or entry points. First locate their references and confirm whether removal belongs in a 6.x patch or 7.0.0; done means the obsolete support and define are removed without breaking supported targets.