The gp_external_grant_privileges GUC was needed before 4.0 to let
non superusers create external tables for gphdfs and http protocols.
This GUC was however deprecated during the 4.3 cycle so remove all
traces of it. The utility of the GUC was replaced in 4.0 when rights
management for external tables was implemented with the normal GRANT
REVOKE framework so this has been dead code for quite some time.

Remove GUC, code which handles it, all references to it from the
documentation and a release notes entry.