pgsql: Clean out column-level pg_init_privs entries when dropping table

Clean out column-level pg_init_privs entries when dropping tables.

DeleteInitPrivs did not get the memo about how, when dropping a
whole object (with subid == 0), you should drop entries relating
to its sub-objects too.  This is visible in the test_pg_dump test
case if one drops the extension at the end: the entry for
        GRANT SELECT(col1) ON regress_pg_dump_table TO public;
was still present in pg_init_privs afterwards, although it was
pointing to a dangling table OID.

Noted while fooling with a fix for REASSIGN OWNED for pg_init_privs
entries.  This bug is aboriginal in the pg_init_privs feature
though, and there seems no reason not to back-patch the fix.

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f3f6a14ce3e982fb7fbe9f4f33134f3f693f4b7e

Modified Files
--------------
src/backend/catalog/dependency.c                    | 21 +++++++++++++++------
.../modules/test_pg_dump/expected/test_pg_dump.out  |  7 +++++++
src/test/modules/test_pg_dump/sql/test_pg_dump.sql  |  5 +++++
3 files changed, 27 insertions(+), 6 deletions(-)