Deprecate colorizeTooltip mixin

[broccolini]

This doesn't appear to be used anywhere and not sure why we would want to use it either.

https://github.com/primer/primer/blob/c865391eaa1aa743f9ca3c24133cc30efa51acbd/modules/primer-tooltips/lib/tooltips.scss#L242-L272

To do:

• Add deprecation warning in minor release
• Remove Sass in major release

[shawnbot]

This was done in #656 and released in #498.