File Editor: Remove CSS exceptions for the "Select plugin/theme to edit" and...

File Editor: Remove CSS exceptions for the "Select plugin/theme to edit" and documentation "Look Up" buttons.

Buttons should use the default buttons styles, no exceptions.

Props passoniate, garethgillman.
See #49197.
Fixes #49353.

Built from https://develop.svn.wordpress.org/trunk@47751


git-svn-id: https://core.svn.wordpress.org/trunk@47527 1a063a9b-81f0-0310-95a4-ce76da25c4cd