Revert "broken link checker: check external links too."

This reverts commit 9e5ec00d.

Unfortunately, we are getting too many timeouts.

package.json

@@ -18,7 +18,7 @@
   ],
   "scripts": {
     "start": "npm-run-all --parallel watch docs-serve",
-    "blc": "blc --filter-level 3 --get --ordered --recursive --host-requests 4 --input http://localhost:3000/",
+    "blc": "blc --exclude-external --filter-level 3 --get --ordered --recursive --host-requests 4 --input http://localhost:3000/",
     "http-server": "http-server --silent -p 3000",
     "bundlesize": "bundlesize",
     "check-broken-links": "npm-run-all --parallel --race \"http-server -- _gh_pages/\" blc",