Remove the APICAST_WORKERS value so that APIcast can auto-detect

APIcast template specifies the number of APICAST_WORKERS and defaults to 1:
https://github.com/3scale/3scale-amp-openshift-templates/blob/2.2.0.GA/apicast-gateway/apicast.yml#L132

This prevents auto-detection of the workers implemented here.

The value should be removed to allow auto-detection, although the parameter can stay in the template so that it can be easily modified.

BTW, amp.yml template doesn't specify this parameter.