From PostgreSQL 13, pg_rewind will automatically handle an unclean
shutdown itself, so as long as --force-rewind was provided, so there
is no need to fail with an error.
Note that pg_rewind handles the unclean shutdown by starting PostgreSQL
in single user mode, which it does before performing any checks as
to whether a rewind is actually necessary.
However pg_rewind doesn't take into account the possible presence
of a standby.signal file, so we remove that and recreate it after
pg_rewind was executed.
