Removed reference to open source browser extension (#710)

Removed reference to the browser extension being open-sourced.

Author: noah-berman

docs/integration/browser_extension/index.mdx

@@ -6,8 +6,7 @@
   <source src="https://storage.googleapis.com/sourcegraph-assets/code-host-integration/CodeIntelligenceOnCodeHostDark.mp4"/>
 </video>
 
-The <a href="https://github.com/sourcegraph/sourcegraph/tree/main/client/browser">open-source</a> Sourcegraph
-browser extension adds code navigation to files and diffs on GitHub, GitHub
+The Sourcegraph browser extension adds code navigation to files and diffs on GitHub, GitHub
 Enterprise, GitLab, Phabricator, Bitbucket Server and Bitbucket Data Center.
 
 - <a href="https://chrome.google.com/webstore/detail/sourcegraph/dgjhfomjieaadpoljlnidmbgkdffpack" target="_blank">Install Sourcegraph for Chrome</a>