Eliminate use of safecss_filter_attr() in processing style attributes since redundant and lossy

[westonruter]

Fixes #613.

There is a problem with the use of safecss_filter_attr() in \AMP_Style_Sanitizer::process_style() introduced via https://github.com/Automattic/amp-wp/blob/23eced628815e169ca9831b2ccb460b49adc53c1/includes/sanitizers/class-amp-style-sanitizer.php#L456

This is what is removing the display property. It was introduced in de2a303 with the comment:

Filter the styles using safecss_filter_attr() to make sure there
aren't any funky properties. Also alphabetize the valid properties so
that variable order doesn't cause unnecessary duplication.

The logic in safecss_filter_attr() is already being run on post save to strip out display, if the user doesn't have unfiltered_html. So this is preventing safecss_filter_attr() from being called a second time when displaying the post, resulting in a plugin that outputs a Kses-illegal style property to be also erroneously stripped. Otherwise, if a user does have unfiltered_html then they currently aren't able to do so in AMP even in spite of the sanitizer.

[coreymckrill]

Looks good to me.