Remove oauth2client from IoT and Functions samples [(#3255)](GoogleCloudPlatform/python-docs-samples#3255)

* chore: remove oauth2client from functions and iot

* fix: fix import

* fix: correct syntax

* fix: correct import for service_account

* fix: fix path

Author: busunkim96

samples/api-client/manager/requirements.txt

@@ -5,6 +5,5 @@ google-auth-httplib2==0.0.3
 google-auth==1.11.2
 google-cloud-iot==1.0.0
 google-cloud-pubsub==1.1.0
-oauth2client==4.1.3
 paho-mqtt==1.5.0
 pyjwt==1.7.1