mediawiki.skinning: Remove styles for `#jump-to-nav` from core

mediawiki.skinning: Remove styles for `#jump-to-nav` from core

There are no longer any skins bundled with MediaWiki releases that
use it. Styling for this should be the skins' responsibility.

Vector and MonoBook no longer use this element, and core's
providing of these styles actually creates conflicts.

For skins using 'mediawiki.legacy.oldshared' (predating MonoBook),
no changes are required! Their jump-to-nav styles remain preserved.

For skins that use 'mediawiki.skinning.interface' and have
MonoBook-inspired accessibility links within a `<div id="#jump-to-nav">`,
need to make one of two changes:

1. Adopt the new CSS-only approach documented in T195256,
   as used by current MonoBook and Vector.

2. Or; Add a copy of the old CSS (included below) to your own skins'
   stylesheet and ensure the `jquery.mw-jump` module is loaded (previously
   by default), either by adding `$out->addModules( 'jquery.mw-jump' )`, or
   by adding it from the skin's Skin::getDefaultModules() override.

    @media screen {
        #jump-to-nav {
            /* Negate #contentSub margin */
            margin-top: -1.4em;
            margin-bottom: 1.4em;
            -moz-user-select: none;
            -webkit-user-select: none;
            -ms-user-select: none;
            user-select: none;
        }
        .mw-jump, #jump-to-nav {
            overflow: hidden;
            height: 0;
            zoom: 1;
        }
    }

    @media print {
        .mw-jump, #jump-to-nav {
            display: none;
        }
    }

This migration guide will be added to the T195256 task description,
to which the release notes refer.

Bug: T195256
Change-Id: I84bcd23180b3d1fa541728989f44a376189df95d