Remove apparent support for SECURITY LABEL ON PROPERTY GRAPH

Remove apparent support for SECURITY LABEL ON PROPERTY GRAPH

Commit 2f094e7ac69 added a mention of SECURITY LABEL ON PROPERTY GRAPH
to the SECURITY LABEL reference page, and it added support to psql tab
completion.  However, security labels on property graphs are not
actually supported (per SecLabelSupportsObjectType()).  The syntax
does work, but that is just a result of how gram.y is factored.  We
don't document or tab-complete the syntax of SECURITY LABEL for other
object types that are not actually supported, so it was inconsistent
to do this for property graphs.  Thus, remove this.

Reported-by: Noah Misch <noah@leadboat.com>
Discussion: https://www.postgresql.org/message-id/flat/20260704221210.08.noahmisch%40microsoft.com