Update postgres_backup.sh to cleanup daily backups

[seppelucas]

Removes safeguard to leave at least as many backups as BACKUP_NUM_TO_RETAIN

Currently when running backups on a schedule any less frequent then daily the backups are not cleaned up as the zalando/postgres-operator documentation suggests. See ticket #425.

This PR fixes #425.

The safeguard was not mentioned in the documentation.

Also: BACKUP_NUM_TO_RETAIN is a number of days i would suggest renaming it to BACKUP_NUM_DAYS_TO_RETAIN it is only used in the zalando/postgres-operator docs. Let me know if that fix is desired. Decided to keep the fix small for now.

Multiple solutions already offered, this would be the most simple one. Also see possible fix in #425.