Don't fill in the skin's search box on Special:Search

Don't fill in the skin's search box on Special:Search

When viewing Special:Search, you usually see two search forms: the
small one provided by the skin (present on all pages), and the large
advanced search form generated by Special:Search.

Previously, both of these forms would be prefilled with the text that
was searched, which could lead to the user submitting the wrong form
after changing advanced search options.

After this change, the skin's search form will always start out empty.
There's no way for this to affect pages other than Special:Search,
since if the 'search' URL parameter is given, all requests are
redirected to Special:Search anyway (see MediaWiki::parseTitle()).

Bug: T106055
Change-Id: Ic6bb8af5383e7acfe4f823f563165cd60182753e