Remove unnedded Active Storage installation steps

Active Storage was now merged to master and it is automatically
installed in a new application.

activestorage/README.md

@@ -83,14 +83,6 @@ Variation of image attachment:
 <%= image_tag user.avatar.variant(resize: "100x100") %>
 ```
 
-## Installation
-
-1. Run `rails activestorage:install` to create needed directories, migrations, and configuration.
-2. Optional: Add `gem "aws-sdk", "~> 2"` to your Gemfile if you want to use AWS S3.
-3. Optional: Add `gem "google-cloud-storage", "~> 1.3"` to your Gemfile if you want to use Google Cloud Storage.
-4. Optional: Add `gem "azure-storage"` to your Gemfile if you want to use Microsoft Azure Storage.
-5. Optional: Add `gem "mini_magick"` to your Gemfile if you want to use variants.
-
 ## Direct uploads
 
 Active Storage, with its included JavaScript library, supports uploading directly from the client to the cloud.