remove gpdb_12_merge_fixme in pg_import_system_collations (#13176)
remove gpdb_12_merge_fixme in pg_import_system_collations (#13176)

* remove gpdb_12_merge_fixme in pg_import_system_collations

Postgres 10 added the pg_import_system_collations() UDF that will
import collations from all system locales installed on the system.
This function is used in initdb to import the collations on the
system, or can be called by a system administrator to import newly
added collations any time thereafter.

Some adaptations were made to MPP during the GreenPlum backport
this UDF in https://github.com/greenplum-db/gpdb/pull/5016.
In order for collations to work correctly, we need to be sure that
every collation that is created on the QD is also installed on the
QEs, and that the OID matches in every database.  In upstream,
collations are created during initdb, but this won't work for GPDB,
because while initdb is running there is no way to be sure that
every segment has the same locales installed.

Since we no longer import collations in initdb and database
administrator need to call this UDF manually after the database is
created, we only need to import collations that are compatible with
the current database encoding in pg_import_system_collations().

* Apply suggestions from code review

Co-authored-by: QingMa <45c5eed0cf5fef5d022ad63b71ca35c0c68845cc@vmware.com>
Co-authored-by: Zhenghua Lyu <51845dc4db1d2d38149a1027f835e70909dc2cb1@gmail.com>
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