[exposure analysis] remove ExternalExposure from the PolicyExposureWithoutSelectors

[shireenf-ibm]

• external-exposure is actually computed and generated using the IPBlock Peer and the ExternalExposure field of the PolicyExposureWithoutSelectors struct is actually not used in the exposure-analysis so better to remove it to have more clean code
• mention in documentation that exposure analysis struct only computes exposure to entire-cluster (for external ip-blocks just copying the conn-list results).