Remove mail blockquote special case handling.

LayoutTests/editing/inserting/5418891.html

-<style>
-blockquote {
-    color: blue;
-    border-left: 2px solid blue;
-    padding-left: 5px;
-    margin: 0px;
-}
-</style>
-<p>This tests for a crash when attempting to break a blockquote at the end of its content.  The caret should be in the first of two empty paragraphs between two pieces of quoted content.</p>
-<div contenteditable="true">
-<blockquote type="cite"><div id="dv">foo</div></blockquote>
-<br>
-<blockquote type="cite"><div>bar</div></blockquote>
-</div>
-
-<script>
-dv = document.getElementById("dv");
-text = dv.firstChild;
-s = window.getSelection();
-s.setPosition(text, 3);
-document.execCommand("InsertNewlineInQuotedContent");
-</script>