Cleanup useless assignments and checks

This cleans up a couple of areas:
- Remove XLogSegNo calculation for the last WAL segment in backup in
xlog.c (7d70809 has moved this logic entirely to xlogbackup.c when
building the contents of the backup history file).
- Remove check on log_min_duration in analyze.c, as it is already true
where this code path is reached.
- Simplify call to find_option() in guc.c.

Author: Ranier Vilela
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/CAEudQArCDQQiPiFR16=yu9k5s2tp4tgEe1U1ZbkW4ofx81AWWQ@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -8733,7 +8733,6 @@ do_pg_backup_stop(BackupState *state, bool waitforarchive)
 		 */
 		RequestXLogSwitch(false);
 
-		XLByteToPrevSeg(state->stoppoint, _logSegNo, wal_segment_size);
 		state->stoptime = (pg_time_t) time(NULL);
 
 		/*