Removed references to usemap, per discussion in issue #29.

Author: Wilto

ResponsiveImages.html

@@ -83,7 +83,6 @@ <h1>The <code>picture</code> element</h1>
     <dd><a href="http://dev.w3.org/html5/spec/single-page.html#phrasing-content-1">Phrasing content</a>.</dd>
     <dd><a href="http://dev.w3.org/html5/spec/single-page.html#embedded-content-2">Embedded content</a>.</dd>
     <dd><a href="http://dev.w3.org/html5/spec/single-page.html#palpable-content-0">Palpable content</a>.</dd>
-    <dd>If the element has a <a href="http://dev.w3.org/html5/spec/image-maps.html#attr-hyperlink-usemap">usemap</a> attribute: <a href="http://dev.w3.org/html5/spec/content-models.html#interactive-content-0">Interactive content</a>.</dd>
     <dt><a href="http://dev.w3.org/html5/spec/single-page.html#element-dfn-contexts" title="element-dfn-contexts">Contexts in which this element can be used</a>:</dt>
     <dd>Where <a href="http://dev.w3.org/html5/spec/single-page.html#embedded-content-2">embedded content</a> is expected.</dd>
     <dt><a href="http://dev.w3.org/html5/spec/single-page.html#element-dfn-content-model" title="element-dfn-content-model">Content model</a>:</dt>
@@ -93,7 +92,6 @@ <h1>The <code>picture</code> element</h1>
     <dt><a href="http://dev.w3.org/html5/spec/single-page.html#element-dfn-attributes" title="element-dfn-attributes">Content attributes</a>:</dt>
     <dd><a href="http://dev.w3.org/html5/spec/single-page.html#global-attributes">Global attributes</a></dd>
     <dd><code>crossorigin</code></dd>
-    <dd><code>usemap</code></dd>
     <dd><code>ismap</code></dd>
     <dd><code>width</code></dd>
     <dd><code>height</code></dd>