MonoBook: Remove usage of $wgUseTwoButtonsSearchForm config variable

The search form no longer changes based on this config variable.

Note that the variable has always been true since its introduction and
there's no way for users to turn it off, so probably the single button
search form has never been shown.

Bug: T256568
Change-Id: I86fde4f89cff553966614d725bbd6ba14d4ab1a4