OPS-976: Remove serial option from mongo play

The mongo play doesn't work when run serially. It's run non-serially in
other places (vagrant-cluster.yml, for example). The serial option seems
to have been added mistakenly in 2632cb86 (#1522) and not tested.

playbooks/edx-east/mongo.yml

@@ -2,9 +2,6 @@
   hosts: all
   sudo: True
   gather_facts: True
-  vars:
-    serial_count: 1
-  serial: "{{ serial_count }}"
   roles:
     - mongo
     - role: datadog