pgsql: Fix properties orphaned by dropping a label

Fix properties orphaned by dropping a label

AlterPropGraph() cleans up pg_propgraph_property entries that are
orphaned by dropping an element or by dropping properties associated
with an element. But it did not clean up pg_propgraph_property
entries that are orphaned by dropping labels associated with an
element. Fix this missing case.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Author: zengman <zengman(at)halodbtech(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/tencent_76F6ACA2364EAA1E5DBD7A47%40qq.com

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/45d4c917ff4f4fe206ad18d0279a47c9f2a08123

Modified Files
--------------
src/backend/commands/propgraphcmds.c | 2 +-
.../regress/expected/create_property_graph.out | 23 +++++++++++-----------
src/test/regress/sql/create_property_graph.sql | 5 +++++
3 files changed, 17 insertions(+), 13 deletions(-)