Twenty Twelve: Remove IE specific checks

[sabernhardt]

Twenty Twelve changes:

• Registers ie.css without enqueuing it
• Removes IE conditional code and HTML5 Shiv from header.php
• Removes width hack from editor-style.css
• Removes extra clip syntax from custom-header.php

Trac 58836

Props: metodiew

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[github-actions]

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[github-actions]

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Props metodiew, sabernhardt.

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