pg_archivecleanup: Refactor loop doing old segment removals
This commit refactors a bit the main loop of pg_archivecleanup that
handles the removal of old segments, reducing by one its level of
indentation. This will help an incoming patch that adds a new option
related to the segment filtering logic.
Author: Atsushi Torikoshi
Reviewed-by: Kyotaro Horiguchi, Fujii Masao, Michael Paquier
Discussion: https://postgr.es/m/
d660ef741ce3d82f3b4283f1cafd576c@oss.nttdata.com