Icon: Remove automatic heuristic-based monochrome icon re-coloring

This feature has caused many bugs over the years and its value is dubious to begin with. Automatically masking a colorful icon to instead be monochrome based on this or that heuristic violates the integrity of the user's icon theme and enforces a stylistic preference for monochrome icons that is not appropriate to do at the library level.

If the user is using an icon theme whose icons have any color in them despite being 16 or 22px size or using the -symbolic suffix, this needs to be considered an intentional stylistic choice on the part of the icon theme, and it isn't something that we should monkey with here.

As a bonus, we also reap an efficiency win by simply doing less unnecessary work most of the time.

If a developer wants to do this masking anyway, they are still free to set mask: true in their code and then it will happen on an opt-in basis.

BUG: 473001 BUG: 471803 BUG: 469978 BUG: 465422 BUG: 451538 FIXED-IN: 6.0