remove goXDCR keys from internalSettings

DOC-1245 - deprecate go_xdcr api's from internalSettings in 4.5

GET/POST goxdcr api's with urls /internalSettings and /settings/replications return and allow to modify the same configuration parameters, which are just default values for replication configuration.

With recent RBAC changes ns_server /internalSettings api that currently encapsulates goxdcr /internalSettings api is available only to built in Administrator. This creates following issue: replication_admin user is not able to modify the defaults via UI, but is able to do it using /settings/replications api.

Here's what I suggest:
1. Remove /internalSettings api from goxdcr
2. So xdcr related settings will no longer appear in Internal Settings dialog

That should not affect customers since Internal Settings are well... internal, so should be used only by recommendation of couchbase tech support.

Customers will still be able to modify defaults using REST API (and CLI?)