servo: Merge #10465 - Fix rebasing error (from stjepang:rescue-jit-prefs); r=KiChjang

This PR mistakenly removed JIT enabling/disabling by preference:
https://github.com/servo/servo/pull/10342/files
Look for `get_pref`.

I'm putting the missing piece of code into the appropriate place in
script_runtime.rs

r? @KiChjang

Source-Repo: https://github.com/servo/servo
Source-Revision: bead9585000eed60394b20c7a8e3788284ea458a

servo/components/script/script_thread.rs

@@ -101,7 +101,6 @@ use task_source::user_interaction::UserInteractionTaskSource;
 use time::Tm;
 use url::Url;
 use util::opts;
-use util::prefs::get_pref;
 use util::str::DOMString;
 use util::thread;
 use util::thread_state;