postgresql: drop per-database autovacuum ALTERs (rejected by Postgres)

Follow-up to the previous two commits: the ALTER DATABASE ... SET
autovacuum_* commands fail at boot with

  ERROR: parameter "autovacuum_vacuum_scale_factor" cannot be changed now

and take matrix-synapse-db-tune.service (and nextcloud-db-init.service)
down with them.

Root cause: ALTER DATABASE/ROLE ... SET validates through
set_config_option() with an interactive context, and guc.c rejects any
PGC_SIGHUP parameter set that way. All autovacuum_* GUCs are
SIGHUP-context, so per-database scoping is impossible for them — only
USERSET-level parameters (e.g. work_mem, statement_timeout) can be set
per-database.

Nothing is lost: the same values are already set cluster-wide in
configuration.nix, which covers both nextclouddb and matrix-synapse.
Remove the ALTERs from nextcloud-db-init and delete the now-purposeless
matrix-synapse-db-tune service.

modules/nextcloud.nix

@@ -60,11 +60,11 @@ lib.mkIf config.sovran_systemsOS.services.nextcloud {
         psql -U postgres -c "CREATE DATABASE nextclouddb WITH OWNER ncusr TEMPLATE template0 LC_COLLATE = 'C' LC_CTYPE = 'C';"
       fi
 
-      # Per-database autovacuum, scoped to nextclouddb only.
-      # The shared matrix-synapse DB keeps the milder cluster defaults.
-      # Fixes Nextcloud 35 pg.dead_tuples warning. Idempotent.
-      psql -U postgres -d nextclouddb -c "ALTER DATABASE nextclouddb SET autovacuum_vacuum_scale_factor = '0.05';"
-      psql -U postgres -d nextclouddb -c "ALTER DATABASE nextclouddb SET autovacuum_analyze_scale_factor = '0.025';"
+      # NOTE: autovacuum GUCs are SIGHUP-context, so they cannot be set
+      # per-database — ALTER DATABASE ... SET rejects them with
+      # 'parameter "..." cannot be changed now'. They are set
+      # cluster-wide in configuration.nix instead, which already covers
+      # both nextclouddb and matrix-synapse.
     '';
   };