doc: remove remnant mention of io.SizedReaderAt from Go 1.7 docs

Updates #15810

Change-Id: I37f14a0ed1f5ac24ea2169a7e65c0469bfddd928
Reviewed-on: https://go-review.googlesource.com/23559Reviewed-by: Michael McGreevy <mcgreevy@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.7.html

@@ -985,15 +985,6 @@ from URLs with empty query strings (like <code>/search?</code>).
 <dl id="os"><a href="/pkg/os/">os</a></dl>
 
 <dd>
-<p>
-The
-<a href="/pkg/os/#File"><code>File</code></a>
-type adds a new
-<a href="/pkg/os/#File.Size"><code>Size</code></a>
-method, so that <code>File</code> implements the new
-<a href="/pkg/io/#SizedReaderAt"><code>SizedReaderAt</code></a> method.
-</p>
-
 <p>
 <a href="/pkg/os/#IsExists"><code>IsExists</code></a> now returns true for <code>syscall.ENOTEMPTY</code>,
 on systems where that error exists.