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/node/chainstate.cpp

@@ -210,7 +210,6 @@ ChainstateLoadResult LoadChainstate(ChainstateManager& chainman, const CacheSize
         // ChainstateManager::ResetChainstates(), reinitialize them here without
         // duplicating the blockindex work above.
         assert(chainman.GetAll().empty());
-        assert(!chainman.IsSnapshotValidated());
 
         chainman.InitializeChainstate(options.mempool);