utils: merkletrie, Simplify DiffTreeContext.

When node.Skip() == true, it means the node and all its descendants should be skipped.
Previously, we were still calling AddRecursiveDelete() on such nodes, which is redundant,
since addRecursive() skips the entire subtree.

This change removes those unnecessary AddRecursiveDelete() calls,
and also adds a missing check to skip the 'from' node in the 'onlyFromRemains' case.

Additionally, prettified the if-else structure in the switch case for better readability.

utils/merkletrie/difftree.go

···
 		case noMoreNoders:
 			return ret, nil
 		case onlyFromRemains:
-			if err = ret.AddRecursiveDelete(from); err != nil {
-				return nil, err
+			if !from.Skip() {
+				if err = ret.AddRecursiveDelete(from); err != nil {
+					return nil, err
+				}
 			}
 			if err = ii.nextFrom(); err != nil {
 				return nil, err
 			}
 		case onlyToRemains:
-			if to.Skip() {
-				if err = ret.AddRecursiveDelete(to); err != nil {
-					return nil, err
-				}
-			} else {
+			if !to.Skip() {
 				if err = ret.AddRecursiveInsert(to); err != nil {
 					return nil, err
 				}
···
 				return nil, err
 			}
 		case bothHaveNodes:
-			if from.Skip() {
-				if err = ret.AddRecursiveDelete(from); err != nil {
-					return nil, err
-				}
+			var err error
+			switch {
+			case from.Skip():
 				if from.Name() == to.Name() {
-					if err := ii.nextBoth(); err != nil {
-						return nil, err
-					}
+					err = ii.nextBoth()
 				} else {
-					if err := ii.nextFrom(); err != nil {
-						return nil, err
-					}
+					err = ii.nextFrom()
 				}
-				break
-			}
-			if to.Skip() {
-				if err = ret.AddRecursiveDelete(to); err != nil {
-					return nil, err
-				}
+			case to.Skip():
 				if from.Name() == to.Name() {
-					if err := ii.nextBoth(); err != nil {
-						return nil, err
-					}
+					err = ii.nextBoth()
 				} else {
-					if err := ii.nextTo(); err != nil {
-						return nil, err
-					}
+					err = ii.nextTo()
 				}
-				break
+			default:
+				err = diffNodes(&ret, ii)
 			}
 
-			if err = diffNodes(&ret, ii); err != nil {
+			if err != nil {
 				return nil, err
 			}
 		default: