Twenty Fourteen: Remove text decoration from button block.

The button block in Twenty Fourteen was incorrectly inheriting a text underline from the theme's link styles.

Props bobbingwide.
Fixes #45732.


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git-svn-id: https://core.svn.wordpress.org/trunk@44208 1a063a9b-81f0-0310-95a4-ce76da25c4cd