Clean up `!important` usage throughout Polaris

[aaronccasanova]

Throughout the process of removing SCSS mixins and replacing tokens we have come across a number of questionable usages of !important. I suggest we time box a day or so to investigate !important usage across Polaris, clean up any low hanging fruit, and determine next steps for any remaining !importants.

[BPScott]

#4995 (comment) offers up some context on the !importants that came from inlining the hidden when printing mixins.

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