redis: handle changes to systemd support

The 6.0 changelog notes that systemd support was rewritten. The effects
of that seem to be twofold:

* Redis will silently fail to sd_notify if not built with libsystemd,
  breaking our unit configuration.
* It also appears to misbehave if told to daemonize when running under
  systemd -- note that upstream's sample unit configuration does not
  daemonize:
  https://github.com/antirez/redis/blob/unstable/utils/systemd-redis_server.service

nixos/modules/services/databases/redis.nix

@@ -11,12 +11,11 @@ let
     port ${toString cfg.port}
     ${condOption "bind" cfg.bind}
     ${condOption "unixsocket" cfg.unixSocket}
-    daemonize yes
+    daemonize no
     supervised systemd
     loglevel ${cfg.logLevel}
     logfile ${cfg.logfile}
     syslog-enabled ${redisBool cfg.syslog}
-    pidfile /run/redis/redis.pid
     databases ${toString cfg.databases}
     ${concatMapStrings (d: "save ${toString (builtins.elemAt d 0)} ${toString (builtins.elemAt d 1)}\n") cfg.save}
     dbfilename dump.rdb