Remove 2.x to 3.x upgrade document

Author: jeromegamez

README.md

@@ -16,7 +16,6 @@ A Laravel package for the [Firebase PHP Admin SDK](https://github.com/kreait/fir
 - [Configuration](#configuration)
 - [Usage](#usage)
   - [Multiple projects](#multiple-projects)
-- [Upgrading](UPGRADE.md)
 - [Support](#support)
 - [License](#license)
 
@@ -58,9 +57,6 @@ $app->register(Kreait\Laravel\Firebase\ServiceProvider::class);
 $app->withFacades();
 ```
 
-## Upgrade
-See [UPGRADE.md](UPGRADE.md) for upgrade instructions.
-
 ## Configuration
 
 In order to access a Firebase project and its related services using a server SDK, requests must be authenticated.