Block backup operations for standbys

Backup from standbys should use a method based on replication protocol
in a way similar to what is done in pg_basebackup, as it cannot use
pg_start/stop_backup. As I am not sure what would be the right approach
by the way, it is better for the time being to block backups taken
from a standby. It does not penalize the functionality though as taking
disk snapshots is not forbidden either, and a user can still recover
from that. This commit removes at the same time some home-made functions
that created custom backup label files, this is not relyable, especially
if Postgres core format for this file changes across versions. Removing
them at least will save from some bugs for sure.