Remove obsolete prefs from meek-http-helper-user.js
I was looking at the HTTP helper and I noticed that we no longer need to override the network.http.spdy.* prefs.
I did a testbuild with this patch on top of 50193b4aff592e79b5a45fccb2a1547cef8e2691, tried meek-azure, and got fingerprint bb94e801f7aee52b as before (as in #26241 (moved))
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