promtail: Use the default journal path configuration

Commit 967a317 set the journal to
`volatile` which makes journald to write the logs to `/var/run/journal`
instead of `/var/log/journal`.  The default settings of the promtail
journal scraper handles both cases:

> Path to a directory to read entries from. Defaults to system
> paths (/var/log/journal and /run/log/journal) when empty.

Reference: https://grafana.com/docs/loki/latest/send-data/promtail/configuration/#journal

modules/promtail.nix

@@ -22,7 +22,6 @@ in
         {
           job_name = "journal";
           journal = {
-            path = "/var/log/journal";
             max_age = "12h";
             labels = {
               job = "systemd-journal";