Bug 1759367: remove customization for color formats, not needed

Customization: Add more legal color formats such as rgba, hsl, and hsla
(Bug 1264098)
The library added support for the color formats in file
htmlpurifier/HTMLPurifier/AttrDef/CSS/Color.php

behatnotneeded

Change-Id: I60a59b25e54a6ad584518a1629450fdf37fbbaab

htdocs/lib/htmlpurifier/README.Mahara

@@ -10,5 +10,4 @@ content.
 Changes:
 
   * Add the configuration directive Filter.ExtractStyleBlocks.PreserveCSS to allow the comments while cleaning CSS
-  * Add more legal color formats such as rgba, hsl, and hsla
   * Altered SafeIframe.php so that an HTTPS site will rewrite HTTP iframes to protocol-relative