Remove archived Slack extension

Repo archived in 2018, predates Slack's API/token changes; no maintained
drop-in replacement.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -167,7 +167,6 @@ out <a href="https://github.com/sindresorhus/awesome">awesome</a>.
   - [carbon-now-sh](#carbon-now-sh)
   - [Quokka](#quokka)
   - [Runner](#runner)
-  - [Slack](#slack)
   - [Spotify](#spotify)
   - [SVG](#svg)
   - [Text Marker (Highlighter)](#text-marker-highlighter)
@@ -1053,12 +1052,6 @@ Example of toggling `typescript.inlayHints.functionLikeReturnTypes.enabled` by s
 
 ![Run various scripts right from VS Code animation](https://raw.githubusercontent.com/mattn/vscode-runner/master/images/screenshot.gif)
 
-## [Slack](https://marketplace.visualstudio.com/items?itemName=sozercan.slack)
-
-> Send messages and code snippets, upload files to Slack
-
-![Send messages or code snippets to Slack animation](https://raw.githubusercontent.com/sozercan/vscode-slack/master/slack-upload.gif)
-
 ## [Spotify](https://marketplace.visualstudio.com/items?itemName=shyykoserhiy.vscode-spotify)
 > Provides integration with Spotify Desktop client. Shows the currently playing song in status bar, search lyrics and provides commands for controlling Spotify with buttons and hotkeys.