Remove separate opacity rule for disabled MenuItem icon.

The icon specific rule was presumably created because apps can specify separate enabled
and disabled icons.  Sometimes a disabled icon would be bleached out or black-and-white,
rather than color, and wouldn't need any CSS help to look "disabled".
Thus the icon opacity was set to 1 whereas the text was opacity:0.5.

However, we do need the icon cell to have the the same opacity:0.5 setting for the
purpose of the hovered blue background and hovered border.

Fixes #17031.