Simplify code in objectaddress.c for some property graph objects

Simplify code in objectaddress.c for some property graph objects

Property graph element labels and label properties relied on a direct
systable scan when retrieving their object descriptions.  These can be
simplified with get_catalog_object_by_oid().  This offers the benefit to
do a direct syscache lookup, if available.

The same logic will be used in a follow-up patch when retrieving the
object identity parts, applying the same rule across the board for these
object types.

Extracted from a larger patch by the author.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Alex Guo <guo.alex.hengchen@gmail.com>
Discussion: https://postgr.es/m/aej1DkLwhyZWmtxJ@bdtpg