Remove doc and code comments about ON CONFLICT deficiencies

They have been fixed, so we don't need this text anymore.  This reverts
commit 8b18ed6dfb.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Discussion: https://postgr.es/m/CADzfLwWo+FV9WSeOah9F1r=4haa6eay1hNvYYy_WfziJeK+aLQ@mail.gmail.com

doc/src/sgml/ref/insert.sgml

@@ -594,15 +594,6 @@ INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replac
     </para>
    </tip>
 
-   <warning>
-    <para>
-      While <command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX
-      CONCURRENTLY</command> is running on a unique index, <command>INSERT
-      ... ON CONFLICT</command> statements on the same table may unexpectedly
-      fail with a unique violation.
-    </para>
-   </warning>
-
   </refsect2>
  </refsect1>
 

src/backend/optimizer/util/plancat.c

@@ -789,11 +789,6 @@ find_relation_notnullatts(PlannerInfo *root, Oid relid)
  * the purposes of inference.  If no opclass (or collation) is specified, then
  * all matching indexes (that may or may not match the default in terms of
  * each attribute opclass/collation) are used for inference.
- *
- * Note: during index CONCURRENTLY operations, different transactions may
- * reference different sets of arbiter indexes. This can lead to false unique
- * constraint violations that wouldn't occur during normal operations.  For
- * more information, see insert.sgml.
  */
 List *
 infer_arbiter_indexes(PlannerInfo *root)