Remove splunkforwarder from splunk playbook

Splunk already monitors its own logs, so installing splunkforwarder is
redundant. Also, the splunkforwarder role only works on ubuntu and the
splunk server runs on ec2linux (which is redhat-based).

playbooks/edx-east/splunk.yml

@@ -10,7 +10,5 @@
     - splunk-server
     - role: datadog
       when: COMMON_ENABLE_DATADOG
-    - role: splunkforwarder
-      when: COMMON_ENABLE_SPLUNKFORWARDER
     - role: newrelic
       when: COMMON_ENABLE_NEWRELIC