[5.0] Remove some unused variables

src/Symfony/Component/HttpClient/Internal/HttplugWaitLoop.php

@@ -72,7 +72,7 @@ final class HttplugWaitLoop
                         goto check_duration;
                     }
 
-                    if ([$request, $promise] = $this->promisePool[$response] ?? null) {
+                    if ([, $promise] = $this->promisePool[$response] ?? null) {
                         unset($this->promisePool[$response]);
                         $promise->resolve($this->createPsr7Response($response, true));
                     }

src/Symfony/Component/String/UnicodeString.php

@@ -243,7 +243,7 @@ class UnicodeString extends AbstractUnicodeString
                 $tail = substr($tail, \strlen($slice) + \strlen($from));
             }
 
-            $str->string = $result .= $tail;
+            $str->string = $result.$tail;
             normalizer_is_normalized($str->string) ?: $str->string = normalizer_normalize($str->string);
 
             if (false === $str->string) {

src/Symfony/Component/VarDumper/Cloner/VarCloner.php

@@ -28,7 +28,6 @@ class VarCloner extends AbstractCloner
         $pos = 0;                       // Number of cloned items past the minimum depth
         $refsCounter = 0;               // Hard references counter
         $queue = [[$var]];    // This breadth-first queue is the return value
-        $indexedArrays = [];       // Map of queue indexes that hold numerically indexed arrays
         $hardRefs = [];            // Map of original zval ids to stub objects
         $objRefs = [];             // Map of original object handles to their stub object counterpart
         $objects = [];             // Keep a ref to objects to ensure their handle cannot be reused while cloning