migration: Clean up around tls_creds, tls_hostname

Optional MigrationParameters members tls_creds and tls_hostname can't
actually be absent outside qmp_migrate_set_parameters() since commit
4af245dc (v2.9.0).

Note that commit 4af245dc reverted the part of commit de63ab61 (v2.8.0)
that made tls_creds and tls_hostname absent instead of "" in the value
of query-migrate-parameters, even though commit de63ab61 called that a
mistake.  What a mess.

Drop the redundant tests for presence, and update documentation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>