fix(material/button): compute M2 icon-button density padding with CSS calc

[jvanderen1]

Summary

• Replaces compile-time Sass math.div padding in the M2 icon-button density theme with native CSS calc() using --mat-icon-button-state-layer-size and --mat-icon-button-icon-size.
• Padding now stays centered when either token changes at runtime, matching the existing structural styles in icon-button.scss.
• Removes obsolete TODOs and the unused sass:math / $icon-size helpers from the density path.

Test plan

• Verify M2 icon buttons at density scales 0 through -5 keep the icon visually centered
• Override --mat-icon-button-state-layer-size and confirm padding updates with width/height
• Override --mat-icon-button-icon-size and confirm padding updates accordingly
• Smoke-check M3 icon buttons (density path unchanged) for no visual regression

[google-cla]

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[jvanderen1]

I went ahead and signed the CLA, but don't know how to re-trigger the check.