Remove pattern="" in the comment form (HTML5 mode) to avoid mistaken :invalid styles.

This was originally included for Opera 10, but is not needed as of Opera 11, and current is Opera 15.

props jorbin, markjaquith.
fixes #24475.



git-svn-id: https://core.svn.wordpress.org/trunk@24573 1a063a9b-81f0-0310-95a4-ce76da25c4cd