Purge old entries more efficiently from ttrss_entries.

Reduces the number of scans on my TT-RSS instance significantly, going
from 312 scans/sec on average to 121 scans/sec.

Query plan before:

QUERY PLAN
Delete on ttrss_entries  (cost=0.00..227810.70 rows=135 width=6)
  ->  Seq Scan on ttrss_entries  (cost=0.00..227810.70 rows=135
width=6)
        Filter: ((SubPlan 1) = 0)
        SubPlan 1
          ->  Aggregate  (cost=8.28..8.29 rows=1 width=4)
                ->  Index Scan using ttrss_user_entries_ref_id_index on
ttrss_user_entries  (cost=0.00..8.27 rows=1 width=4)
                      Index Cond: (ref_id = ttrss_entries.id)

Query plan after:

QUERY PLAN
Delete on ttrss_entries  (cost=807.93..4640.85 rows=13517 width=6)
  ->  Seq Scan on ttrss_entries  (cost=807.93..4640.85 rows=13517
width=6)
        Filter: (NOT (hashed SubPlan 1))
        SubPlan 1
          ->  Seq Scan on ttrss_user_entries  (cost=0.00..740.34
rows=27034 width=4)

(instance has 27K entries).

Notice the total cost which is much lower for the new query.