Do not check for existence of key prior to deletion

We only need to test that the return value from `removeItem()` is
non-null. A `null` value indicates an error, while true/false indicates
the operation occurred; different adapters will return different values
depending on whether the key existed, so a boolean value always
indicates the key no longer exists in the storage.

src/Psr/SimpleCache/SimpleCacheDecorator.php

@@ -135,12 +135,8 @@ public function delete($key)
     {
         $this->validateKey($key);
 
-        if (! $this->storage->hasItem($key)) {
-            return true;
-        }
-
         try {
-            return (bool) $this->storage->removeItem($key);
+            return null !== $this->storage->removeItem($key);
         } catch (Throwable $e) {
             return false;
         } catch (Exception $e) {

test/Psr/SimpleCache/SimpleCacheDecoratorTest.php

@@ -346,22 +346,19 @@ public function testSetShouldReRaiseExceptionThrownByStorage()
 
     public function testDeleteShouldProxyToStorage()
     {
-        $this->storage->hasItem('key')->willReturn(true);
         $this->storage->removeItem('key')->willReturn(true);
         $this->assertTrue($this->cache->delete('key'));
     }
 
     public function testDeleteShouldReturnTrueWhenItemDoesNotExist()
     {
-        $this->storage->hasItem('key')->willReturn(false);
-        $this->storage->removeItem('key')->shouldNotBeCalled();
+        $this->storage->removeItem('key')->willReturn(false);
         $this->assertTrue($this->cache->delete('key'));
     }
 
     public function testDeleteShouldReturnFalseWhenExceptionThrownByStorage()
     {
         $exception = new Exception\ExtensionNotLoadedException('failure', 500);
-        $this->storage->hasItem('key')->willReturn(true);
         $this->storage->removeItem('key')->willThrow($exception);
 
         $this->assertFalse($this->cache->delete('key'));