Removed shareByDefault() implementation

From what I can tell, it was added for purposes of testing; I updated
the test to use `assertAttributeSame`, varying the property to test
against based on SM version.

src/InputFilterPluginManager.php

@@ -158,9 +158,4 @@ public function validatePlugin($plugin)
             throw new Exception\RuntimeException($e->getMessage(), $e->getCode(), $e);
         }
     }
-
-    public function shareByDefault()
-    {
-        return $this->sharedByDefault;
-    }
 }

test/InputFilterPluginManagerTest.php

@@ -52,7 +52,11 @@ public function testIsASubclassOfAbstractPluginManager()
 
     public function testIsNotSharedByDefault()
     {
-        $this->assertFalse($this->manager->shareByDefault());
+        $property = method_exists($this->manager, 'configure')
+            ? 'sharedByDefault' // v3
+            : 'shareByDefault'; // v2
+
+        $this->assertAttributeSame(false, $property, $this->manager);
     }
 
     public function testRegisteringInvalidElementRaisesException()