Merge pull request #4917 from segmentio/remove-xid

Remove cross_domain_id reference

Author: rchinn1

src/unify/identity-resolution/externalids.md

@@ -27,7 +27,6 @@ Segment automatically promotes the following traits and IDs in track and identif
 | android.idfa       | context.device.advertisingId when context.device.type = 'android' AND context.device.adTrackingEnabled = true |
 | android.push_token | context.device.token when context.device.type = 'android'                                                     |
 | anonymous_id       | anonymousId                                                                                                   |
-| cross_domain_id    | cross_domain_id when XID has been enabled for the workspace                                                   |
 | ga_client_id       | context.integrations['Google Analytics'].clientId when explicitly captured by users                           |
 | group_id           | groupId                                                                                                       |
 | ios.id             | context.device.id when context.device.type = 'ios'                                                            |