[Performance] Avoid ReflectionProvider check function exists on NameImportingPostRector

[samsonasik]

While I was trying to fix conflict defined function to fqcn function at PR:

• [AutoImport] Skip conflict defined function to fqcn function #5082

the ReflectionProvider check function is not needed, it seems for native function, checked at

rector-src/rules/CodingStyle/Node/NameImporter.php

Lines 119 to 121 in 50e589c

	if ($name->getAttribute(AttributeKey::IS_FUNCCALL_NAME) === true) {
	return count($name->getParts()) === 1;
	}

[samsonasik]

All checks have passed 🎉 @TomasVotruba I am merging it ;)