Remove hosted slack plugin stub (#1305)

Co-authored-by: Claude <noreply@anthropic.com>

external_plugins/slack/.claude-plugin/plugin.json

@@ -1,7 +0,0 @@
-{
-  "name": "slack",
-  "description": "Slack workspace integration. Search messages, access channels, read threads, and stay connected with your team's communications while coding. Find relevant discussions and context quickly.",
-  "author": {
-    "name": "Slack"
-  }
-}

external_plugins/slack/.mcp.json

@@ -1,10 +0,0 @@
-{
-  "slack": {
-    "type": "http",
-    "url": "https://mcp.slack.com/mcp",
-    "oauth": {
-      "clientId": "1601185624273.8899143856786",
-      "callbackPort": 3118
-    }
-  }
-}