Revert "Set better Cache-Control header for static content"

On furher investigation it seems like all that did was cause unintuitive
behavior. The emoji request flood that was the reason for introducing it
isn't really that big of a deal either, since Plug.Static only needs to
read file modification time and size to determine the ETag.

Closes #1613

test/plugs/cache_control_test.exs

@@ -9,7 +9,7 @@ defmodule Pleroma.Web.CacheControlTest do
   test "Verify Cache-Control header on static assets", %{conn: conn} do
     conn = get(conn, "/index.html")
 
-    assert Conn.get_resp_header(conn, "cache-control") == ["public max-age=86400 must-revalidate"]
+    assert Conn.get_resp_header(conn, "cache-control") == ["public, no-cache"]
   end
 
   test "Verify Cache-Control header on the API", %{conn: conn} do