Set pg_setting.pending_restart when pertinent config lines are removed

Set pg_setting.pending_restart when pertinent config lines are removed

This changes the behavior of examining the pg_file_settings view after
changing a config option that requires restart.  The user needs to know
that any change of such options does not take effect until a restart,
and this worked correctly if the line is edited without removing it.
However, for the case where the line is removed altogether, the flag
doesn't get set, because a flag was only set in set_config_option, but
that's not called for lines removed.  Repair.

(Ref.: commits 62d16c7fc561 and a486e35706ea)

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202107262302.xsfdfc5sb7sh@alvherre.pgsql