Unconditionally disable find-bar transition animation

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1813140

This changes the animation of the Ctrl+F find bar depending on prefers-reduced-motion, which should be available to the chrome context regardless of RFP. I'm not sure if this could differentiate such a users, even when using letterboxing. For example, by timing when letterboxing jump happens vs when Esc is pressed. Although this seems quite specific and relies on user interaction.

However, I'm also wondering whether we want to disable this animation regardless when using letterboxing, if it improves the experience. Currently, whilst using letterboxing, there is a slight jerk in the page position when opening the find bar and then a second jerk as the page settles. May be unrelated.

/cc @ma1