Dashboard columns:

* Improve responsive breakpoints for.
* Remove obsolete network admin option.
* Remove unnecessary JS.

props SergeyBiryukov, kraftbj, lessbloat. fixes #26074.

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git-svn-id: http://core.svn.wordpress.org/trunk@26292 1a063a9b-81f0-0310-95a4-ce76da25c4cd