refactor: Delete ChainstateManager::IsSnapshotValidated() method

IsSnapshotValidated() is only called one place outside of tests, and is use
redundantly in some tests, asserting that a snapshot is not validated when a
snapshot chainstate does not even exist. Simplify by dropping the method and
checking Chainstate m_assumeutxo field directly.

src/validation.cpp

@@ -6367,7 +6367,7 @@ void ChainstateManager::RecalculateBestHeader()
 bool ChainstateManager::ValidatedSnapshotCleanup(Chainstate& validated_cs, Chainstate& unvalidated_cs)
 {
     AssertLockHeld(::cs_main);
-    if (!this->IsSnapshotValidated()) {
+    if (unvalidated_cs.m_assumeutxo != Assumeutxo::VALIDATED) {
         // No need to clean up.
         return false;
     }