Make crash recovery ignore recovery target settings.

In v11 or before, recovery target settings could not take effect in
crash recovery because they are specified in recovery.conf and
crash recovery always starts without recovery.conf. But commit
2dedf4d9 integrated recovery.conf into postgresql.conf and
which unexpectedly allowed recovery target settings to take effect
even in crash recovery. This is definitely not good behavior.

To fix the issue, this commit makes crash recovery always ignore
recovery target settings.

Back-patch to v12.

Author: Peter Eisentraut
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/e445616d-023e-a268-8aa1-67b8b335340c@pgmasters.net