Remove useless casting to same type

Remove useless casting to same type

This removes some casts where the input already has the same type as
the type specified by the cast.  Their presence could cause risks of
hiding actual type mismatches in the future or silently discarding
qualifiers.  It also improves readability.  Same kind of idea as
7f798aca1d5 and ef8fe693606.  (This does not change all such
instances, but only those hand-picked by the author.)

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/aSQy2JawavlVlEB0%40ip-10-97-1-34.eu-west-3.compute.internal