Remove obsolete block-all-mixed-content

This was removed in all major browsers with no replacement.
See https://www.w3.org/TR/mixed-content/#strict-checking for details on the obsolescence.

lib/philomena_web/plugs/content_security_policy_plug.ex

@@ -34,8 +34,7 @@ defmodule PhilomenaWeb.ContentSecurityPolicyPlug do
         {:form_action, ["'self'"]},
         {:manifest_src, ["'self'"]},
         {:img_src, ["'self'", "blob:", "data:", cdn_uri, camo_uri]},
-        {:media_src, ["'self'", "blob:", "data:", cdn_uri, camo_uri]},
-        {:block_all_mixed_content, []}
+        {:media_src, ["'self'", "blob:", "data:", cdn_uri, camo_uri]}
       ]
 
       csp_value =