[swiftlint] Fix violation: redundant_discardable_let

warning: Redundant Discardable Let Violation: Prefer `_ = foo()` over `let _ = foo()` when discarding a result from a function.

Author: jawwad

Breadth-First Search/Tests/Graph.swift

@@ -86,7 +86,7 @@ public class Graph: CustomStringConvertible, Equatable {
     let duplicated = Graph()
 
     for node in nodes {
-      let _ = duplicated.addNode(node.label)
+      _ = duplicated.addNode(node.label)
     }
 
     for node in nodes {

Palindromes/Test/Test/Test.swift

@@ -29,9 +29,9 @@ class PalindromeTests: XCTestCase {
         // This is an example of a performance test case.
         self.measure {
             // Put the code you want to measure the time of here.
-            let _ = isPalindrome("abbcbba")
-            let _ = isPalindrome("asdkfaksjdfasjkdfhaslkjdfakjsdfhakljsdhflkjasdfhkasdjhfklajsdfhkljasdf")
-            let _ = isPalindrome("abababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababa")
+            _ = isPalindrome("abbcbba")
+            _ = isPalindrome("asdkfaksjdfasjkdfhaslkjdfakjsdfhakljsdhflkjasdfhkasdjhfklajsdfhkljasdf")
+            _ = isPalindrome("abababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababa")
         }
     }
 

Treap/TreapCollectionType.swift

@@ -50,7 +50,7 @@ extension Treap: MutableCollection {
 
     mutating set {
       guard let value = newValue else {
-        let _ = try? self.delete(key: key)
+        _ = try? self.delete(key: key)
         return
       }