doc: Remove incorrect description about dropping replication slots.

doc: Remove incorrect description about dropping replication slots.

pg_drop_replication_slot() can drop replication slots created on
a different database than the one where it is executed. This behavior
has been in place since PostgreSQL 9.4, when pg_drop_replication_slot()
was introduced.

However, commit ff539d mistakenly added the following incorrect
description in the documentation:

     For logical slots, this must be called when connected to
     the same database the slot was created on.

This commit removes that incorrect statement. A similar mistake was
also present in the documentation for the DROP_REPLICATION_SLOT
command, which has now been corrected as well.

Back-patch to all supported versions.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: http://postgr.es/m/OSCPR01MB14966C6BE304B5BB2E58D4009F5DE2@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 13