Change imagemagick optional to depend on one variable (alshedivat#745)

Author: rohandebsarkar

_config.yml

@@ -367,8 +367,6 @@ filtered_bibtex_keywords: [abbr, abstract, arxiv, bibtex_show, html, pdf, select
 # Responsive WebP Images
 # -----------------------------------------------------------------------------
 
-enable_responsive_images: true # enables responsive images for your site (recomended, see https://github.com/alshedivat/al-folio/issues/537)
-
 imagemagick:
   enabled: true # enables responsive images for your site (recommended, see https://github.com/alshedivat/al-folio/issues/537)
   widths: