Removed $Date$ tags from docs.

The tags were not being updated for a long time now and were
visible in the final docs.

Reported in https://github.com/boostorg/website/issues/762.

doc/iterator_traits.html

@@ -7,7 +7,6 @@
 <title>Iterator Traits</title>
 <meta name="author" content="David Abrahams" />
 <meta name="organization" content="Boost Consulting" />
-<meta name="date" content="$Date$" />
 <meta name="copyright" content="Copyright David Abrahams 2004." />
 <link rel="stylesheet" href="../../../rst.css" type="text/css" />
 </head>
@@ -24,8 +23,6 @@
 <td><a class="first last reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a></td></tr>
 <tr><th class="docinfo-name">Organization:</th>
 <td><a class="first last reference external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
-<tr><th class="docinfo-name">Date:</th>
-<td>$Date$</td></tr>
 <tr><th class="docinfo-name">Copyright:</th>
 <td>Copyright David Abrahams 2004.</td></tr>
 </tbody>