Remove client_secret property
Remove client_secret property

This property doesn't appear to be used anywhere and we shouldn't expose
credentials without good reason.

If it was in use it should be in there:
https://github.com/cloudfoundry/cf-acceptance-tests/blob/master/helpers/config/config_struct.go

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf