Move main node cleanup back

Summary: If someone wants the node to be cleared out, they will have to
do it themselves.

Auditors: alpert

katex.js

@@ -248,7 +248,6 @@ var clearNode = function(node) {
 };
 
 var process = function(toParse, baseNode) {
-    clearNode(baseNode);
     var tree = parseTree(toParse);
 
     var style = Style.TEXT;
@@ -256,6 +255,7 @@ var process = function(toParse, baseNode) {
     var span = makeSpan(style.cls(), expression);
     var katexNode = makeSpan("katex", [span]);
 
+    clearNode(baseNode);
     baseNode.appendChild(katexNode);
 };