docs: remove github package registry from installation instructions

- We no longer publish ADM to GitHub
- ADM is only published to the npm registry

Author: seansica

README.md

@@ -25,16 +25,12 @@ This site is dynamically generated from the contents of the `@latest` distributi
 
 ## Installation
 
-The ADM is available on both the npm registry and the GitHub package registry.
-
 To install from the npm registry, simply run:
 
 ```bash
 npm install @mitre-attack/attack-data-model
 ```
 
-See [USAGE.md](./docs/USAGE.md#installing-from-github-package-registry) for instructions on how to install from the GitHub package registry.
-
 ## ATT&CK Specification
 
 The ADM is built upon the MITRE ATT&CK® Specification, which formally defines the structure, properties, and relationships of ATT&CK objects. The ATT&CK Specification serves as the authoritative source for how ATT&CK data should be represented and interacted with.