[#25400] YSQL: remove some outdated YB_TODOs

Summary:
- Remove outdated YB_TODO in genam.c:
  - Commit b263910 moves ybctid out of
    ItemPointer.  RelationGetIndexScan tries to clear PG ctid and
    ybctid. ybctid is moved to xs_itup (IndexTuple) and xs_hitup
    (HeapTuple), but both are already set to NULL, so there is no extra
    clearing necessary.
- Remove outdated YB_TODO in indexcmds.c:
  - safe_index is more suited towards PG's CREATE INDEX CONCURRENTLY.
    It can make some sense for YB's, but given YB doesn't support
    transactional DDL and transaction blocks currently adopt a fixed
    catalog version for the entire transaction, YB is far from being
    able to use safe_index at the moment.  So ignore it for the YB code
    path.
  - pgstat_* is already properly merged.  Most of the code was merged in
    by YB ced7c32.  It explicitly
    avoids the PG phases PROGRESS_CREATEIDX_PHASE_WAIT_* and introduces
    YB phases.  PG 92942642788c9d73e4c090ee0a77603f7afbc1d7 is related
    to the PG phase, so it is irrelevant for YB.
Jira: DB-14631

Test Plan:
Close: #25400
Jenkins: skip

Reviewers: fizaa

Reviewed By: fizaa

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40785

Author: jaki

src/postgres/src/backend/access/index/genam.c

@@ -133,10 +133,6 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 	 * Upstream PG commit c2fe139c201c48f1133e9fbea2dd99b8efe2fadd removes
 	 * setting the item pointer invalid.  Bring that back for the sake of YB
 	 * asserts that that PG field is not changed in YB logic.
-	 *
-	 * YB_TODO(jason): the intention of this is to set both PG and YB fields
-	 * invalid, so watch out in case we decide to change the definition of this
-	 * macro to only set the PG fields invalid.
 	 */
 	ItemPointerSetInvalid(&scan->xs_heaptid);
 	scan->yb_exec_params = NULL;

src/postgres/src/backend/commands/indexcmds.c

@@ -533,9 +533,6 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)
  * 'quiet': suppress the NOTICE chatter ordinarily provided for constraints.
  *
  * Returns the object address of the created index.
- *
- * YB_TODO(later): "safe_index" and calls to "pgstat_*()" are new in PG13.
- * Need to check if these changes are also necessary for Yugabyte.
  */
 ObjectAddress
 DefineIndex(Oid relationId,