pgsql: Use USECS_PER_SEC from datatype/timestamp.h

Use USECS_PER_SEC from datatype/timestamp.h

We had two places defining their own constants for this.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Discussion: https://postgr.es/m/202510311750.mxiykx3tp4mx@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ad1581d7feaeb1d78a0858703dac1bcb52f600d8

Modified Files
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src/bin/pg_basebackup/pg_createsubscriber.c |  5 ++---
src/bin/pg_ctl/pg_ctl.c                     | 13 +++++++------
2 files changed, 9 insertions(+), 9 deletions(-)