Administration: Ensure that disabled buttons look disabled.

This change removes the disabled state from the admin scheme specific styles.

Fixes #48709.

Props drw158, SergeyBiryukov, afercia, kennethroberson5556, melchoyce, joedolson, valentinbora, audrasjb, larrach, nrqsnchz, youknowriad.


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git-svn-id: https://core.svn.wordpress.org/trunk@48129 1a063a9b-81f0-0310-95a4-ce76da25c4cd