Revert "Restore search box tabindex"

This does not restore the same behavior as collapsibleNav.js had.  That
used $( document ).lastTabIndex() + 1 in JavaScript.  This means it
would dynamically put the search box's tabindex after other elements.

This commit specified an explicit value of 1.  This is normally used
for forms.  For example, 1 is the username field on Special:UserLogin.
Thus, this made tabbing from the username field take you to the search
input.  It also caused issues with at least MassMessage.

Another solution is necessary, whether it's dynamically adding it with
JavaScript as before, using a different PHP-generated tabindex, both
(PHP-generated tabindex overridden by JavaScript), or something else.

In the meantime, this reverts to avoid the mentioned regressions.

This reverts commit be76751e8c7a8c33b8a9f41faba78592a22ded09.

Bug: 29199
Bug: 65704
Change-Id: Iaea60834aa8f1564aca50c21ee62ae01f958cc65
(cherry picked from commit c8eb7b28a055ba4dd5f47738eb1e1aa790a80bbe)