remove unexisting "productVersion" field from APIManager

Since 3scale 2.6 the "productVersion" field does not exist anymore as an APIManager field.
In 2.8 documentation I've seen that in an example yaml APIManager content the "productVersion" field appears:
https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.8/html-single/installing_3scale/index#deploying-apimanager-custom-resource

That is incorrect and should be removed from there and in any APIManager yaml content that appears or any reference to that field as an APIManager field.

This should be fixed for 3scale 2.6, 2.7, 2.8