Revert "Added override for `NSMutableURLRequest` for the `URLRequestConvertible` protocol."

This reverts commit 30a55d6cceff8c791287871eb0197cb42254502f.

Source/Alamofire.swift

@@ -81,12 +81,6 @@ extension NSURLRequest: URLRequestConvertible {
     }
 }
 
-extension NSMutableURLRequest {
-    public override var URLRequest: NSMutableURLRequest {
-        return self
-    }
-}
-
 // MARK: - Convenience
 
 func URLRequest(