Removed nil check that is no longer needed to satisfy test

Test was updated in google#3244 to test the correct behaviour, which means that we don't need to worry here about whether the tree exists or not.

storage/mysql/tree_storage.go

@@ -160,16 +160,11 @@ func (m *mySQLTreeStorage) beginTreeTx(ctx context.Context, tree *trillian.Tree,
 		return treeTX{}, err
 	}
 	var subtreeRevisions bool
-	if tree.StorageSettings != nil {
-		// This nil check always passes when we have read the tree from the database.
-		// This check avoids a weird test case in logtests.go that uses an uninitialized tree.
-		// TODO(mhutchinson): can the test be changed, or is this uninitialized case legit?
-		o := &mysqlpb.StorageOptions{}
-		if err := anypb.UnmarshalTo(tree.StorageSettings, o, proto.UnmarshalOptions{}); err != nil {
-			return treeTX{}, fmt.Errorf("failed to unmarshal StorageSettings: %v", err)
-		}
-		subtreeRevisions = o.SubtreeRevisions
+	o := &mysqlpb.StorageOptions{}
+	if err := anypb.UnmarshalTo(tree.StorageSettings, o, proto.UnmarshalOptions{}); err != nil {
+		return treeTX{}, fmt.Errorf("failed to unmarshal StorageSettings: %v", err)
 	}
+	subtreeRevisions = o.SubtreeRevisions
 	return treeTX{
 		tx:            t,
 		mu:            &sync.Mutex{},