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/commands/analyze.c

@@ -360,8 +360,7 @@ do_analyze_rel(Relation onerel, VacuumParams *params,
 		}
 
 		pg_rusage_init(&ru0);
-		if (params->log_min_duration >= 0)
-			starttime = GetCurrentTimestamp();
+		starttime = GetCurrentTimestamp();
 	}
 
 	/*