Remove reference to the asdf-semgrep plugin (#1916)

The asdf-semgrep plugin has been non-functional since March 2023.

Author: jkinsfather

docs/extensions/overview.md

@@ -52,10 +52,6 @@ If you would like to run the pre-commit hook **locally** while using Semgrep Pro
 
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-### Version management
-
-- asdf: [ASDF Semgrep](https://github.com/brentjanderson/asdf-semgrep)
-
 ### Semgrep as an engine
 
 Many other tools have capabilities powered by Semgrep.