Removed the Profiler screenshot capture feature

This wasn't reliable because of browser extension API limitations and required serious throttling to avoid harming performance, so I've decided to just remove it entirely for now.

shells/browser/shared/src/contentScript.js

@@ -75,15 +75,3 @@ if (!backendInitialized) {
     }
   }, 500);
 }
-
-chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
-  if (request.event === 'screenshotCaptured') {
-    window.postMessage(
-      {
-        source: 'react-devtools-content-script',
-        payload: request,
-      },
-      '*'
-    );
-  }
-});