removed unecessary checks

Author: S-M-J-I

src/main/java/com/thealgorithms/stacks/GreatestElementConstantTime.java

@@ -37,7 +37,7 @@ public void push(int data) {
         }
 
         mainStack.push(data);
-        if (!maxStack.isEmpty() && data > maxStack.peek()) {
+        if (data > maxStack.peek()) {
             maxStack.push(data);
         }
     }
@@ -55,7 +55,7 @@ public void pop() {
         }
 
         int ele = mainStack.pop();
-        if (!maxStack.isEmpty() && ele == maxStack.peek()) {
+        if (ele == maxStack.peek()) {
             maxStack.pop();
         }
     }

src/main/java/com/thealgorithms/stacks/SmallestElementConstantTime.java

@@ -35,7 +35,7 @@ public void push(int data) {
         }
 
         mainStack.push(data);
-        if (!minStack.isEmpty() && data < minStack.peek()) {
+        if (data < minStack.peek()) {
             minStack.push(data);
         }
     }
@@ -53,7 +53,7 @@ public void pop() {
         }
 
         int ele = mainStack.pop();
-        if (!minStack.isEmpty() && ele == minStack.peek()) {
+        if (ele == minStack.peek()) {
             minStack.pop();
         }
     }