PHPStan: don't remember possibly-impure function return values

I don't think we get much benefit from this, and the assumption that functions with a return value are pure is sketchy.
In any case, it's better to avoid these repeated calls anyway.

phpstan.neon.dist

@@ -19,6 +19,7 @@ rules:
 parameters:
 	level: 9
 	checkMissingCallableSignature: true
+	rememberPossiblyImpureFunctionValues: false #risky to remember these, better for performance to avoid repeated calls anyway
 	treatPhpDocTypesAsCertain: false
 	bootstrapFiles:
 		- tests/phpstan/bootstrap.php