Remove soil_caption. Use WP native HTML5 markup for captions instead

Wordpress already supports HTML markup for captions via `add_theme_support('html5', array('caption'))`. This commit removes the custom caption shortcode filter in Soil

This pull request is associated with a [Roots pull request](https://github.com/roots/roots/pull/1105) that adds in HTML5 captions via `add_theme_support` and applies the caption padding styles from Bootstrap via LESS.