Drop _Join collection when deleting a relation field

src/ExportAdapter.js

@@ -43,8 +43,6 @@ ExportAdapter.prototype.connect = function() {
 
 // Returns a promise for a Mongo collection.
 // Generally just for internal use.
-var joinRegex = /^_Join:[A-Za-z0-9_]+:[A-Za-z0-9_]+/;
-var otherRegex = /^[A-Za-z][A-Za-z0-9_]*$/;
 ExportAdapter.prototype.collection = function(className) {
   if (!Schema.classNameIsValid(className)) {
     throw new Parse.Error(Parse.Error.INVALID_CLASS_NAME,