Editorial: Remove unneccessary prose for legacy aliases

Since speced/bikeshed#1021 is fixed,
it is no longer necessary to `<dfn>` these in prose. Just the IDL
is clear enough, and has a link to WebIDL for LegacyWindowAlias.

Author: zcorpan

geometry/Overview.bs

@@ -215,9 +215,6 @@ and returns a new {{DOMPoint}} object.
  the attribute values <code>x: 20</code> and <code>y: 18</code>.
 </div>
 
-<p>For historical reasons, the {{DOMPoint}} interface has the legacy window alias <dfn
-dfn-type=interface><code>SVGPoint</code></dfn>.
-
 
 <h2 id=DOMRect>The DOMRect interfaces</h2>
 
@@ -375,9 +372,6 @@ steps:
  coordinate</a>, <a for=rectangle>x coordinate</a> + <a for=rectangle>width dimension</a>).
 </div>
 
-<p>For historical reasons, the {{DOMRect}} interface has the legacy window alias <dfn
-dfn-type=interface><code>SVGRect</code></dfn>.
-
 
 <h2 id=DOMQuad>The DOMQuad interface</h2>
 
@@ -809,10 +803,6 @@ prose.
 <p>The {{DOMMatrix}} and {{DOMMatrixReadOnly}} interfaces replace the <code>SVGMatrix</code>
 interface from SVG. [[SVG11]]
 
-<p>For historical reasons, the {{DOMMatrix}} interface has the legacy window aliases <dfn
-dfn-type=interface><code>SVGMatrix</code></dfn> and <dfn
-dfn-type=interface><code>WebKitCSSMatrix</code></dfn>.
-
 
 <h3 id=dommatrixinit-dictionary>DOMMatrixInit dictionary</h3>