rm checks to disallow adding middleware post-init

https://segment.atlassian.net/browse/LIB-1749

Author: Matthew Ault

lib/analytics.js

@@ -105,11 +105,6 @@ Analytics.prototype.addIntegration = function(Integration) {
  */
 
 Analytics.prototype.addSourceMiddleware = function(middleware) {
-  if (this.initialized)
-    throw new Error(
-      'attempted to add a source middleware after initialization'
-    );
-
   this._sourceMiddlewares.add(middleware);
   return this;
 };
@@ -122,11 +117,6 @@ Analytics.prototype.addSourceMiddleware = function(middleware) {
  */
 
 Analytics.prototype.addIntegrationMiddleware = function(middleware) {
-  if (this.initialized)
-    throw new Error(
-      'attempted to add an integration middleware after initialization'
-    );
-
   this._integrationMiddlewares.add(middleware);
   return this;
 };