subscription: Remove unnecessary select_related fields and clean up. #470

Job	Run time
Ubuntu 22.04 production build	5m 19s
Ubuntu 22.04 production install and PostgreSQL upgrade with pgroonga	13m 4s
6.0 Version Upgrade	10m 17s
Ubuntu 24.04 production install	12m 5s
7.0 Version Upgrade	9m 56s
Debian 12 production install with custom db name and user	10m 14s
8.0 Version Upgrade	9m 33s
9.0 Version Upgrade	10m 49s
	1h 21m 17s

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