pgsql: Fix some comments referring to past features

Fix some comments referring to past features

Timestamp can only be an int64 since b9d092c, and support for WITH OIDS
has been removed as of 578b229.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200612023709.GC14879@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7a3543c2ea23d032865f207c2605942b7a32e9ba

Modified Files
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src/backend/access/common/tupdesc.c |  4 +---
src/backend/commands/tablecmds.c    | 14 ++++++--------
src/backend/utils/adt/selfuncs.c    |  4 ++--
3 files changed, 9 insertions(+), 13 deletions(-)