Twenty Seventeen: Ensure header text color updates in Customizer preview when cleared

Checks if the text color has been cleared and removes the `style` block if it has been. Uses color styles in theme stylesheet.

Props sstoqnov, davidakennedy.

Fixes #38993.

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git-svn-id: https://core.svn.wordpress.org/trunk@39387 1a063a9b-81f0-0310-95a4-ce76da25c4cd