box: disable auto schema upgrade

box.schema.upgrade is called automatically by box.cfg if the instance
is rw and box.cfg.replication is unset. This may be unexpected and
dangerous (the upgrade operation is irreversible). Let's drop auto
schema upgrade.

A look into the history of auto schema upgrade:

The feature was first introduced by commit 29a197cf ("box: auto
upgrade to 1.7.5") to automatically create the _truncate system space so
that space.truncate continues to work after update. Later on, we figured
out that auto-upgrade shouldn't be called if the instance is read-only,
see commit 6cfefcd4 ("Do not auto upgrade schema if read only"), or
configures replication, see commit 582a85d4 ("box: disable schema
auto upgrade for replication"). This turned this initially neat and
simple feature into a monstrous creature. We should have zapped it long
time ago.

Closes #8207

NO_DOC=auto schema upgrade wasn't documented