Quick cleanup of chrome_proxy webdriver test

This flag is no longer used on stable or later versions.

TBR=ryansturm@chromium.org

Change-Id: I1919d7015bf0caf8f72b31ce8c80f29f7a2048d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986063Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728665}

tools/chrome_proxy/webdriver/https_previews.py

@@ -24,7 +24,6 @@ class HttpsPreviews(IntegrationTest):
   def EnableLitePageServerPreviewsAndInit(self, t):
     t.EnableChromeFeature('Previews')
     t.EnableChromeFeature('LitePageServerPreviews')
-    t.EnableChromeFeature('HTTPSServerPreviewsUsingURLLoader')
 
     # RLH and NoScript may disable use of LitePageRedirect Previews.
     t.DisableChromeFeature('ResourceLoadingHints')