fix(celery): remove concurrency from the solo worker

Seen in #3738, but I don't think this is the root cause.

Author: nijel

start

@@ -306,7 +306,7 @@ set_real_ip_from 0.0.0.0/0;
     : "${CELERY_MEMORY_OPTIONS:="--concurrency $((WEBLATE_WORKERS == 1 ? 1 : WEBLATE_WORKERS / 2))"}"
     : "${CELERY_BACKUP_OPTIONS:="--concurrency 1"}"
     : "${CELERY_BEAT_OPTIONS:=""}"
-    : "${CELERY_SINGLE_OPTIONS:="--concurrency $WEBLATE_WORKERS"}"
+    : "${CELERY_SINGLE_OPTIONS:=""}"
     : "${WEB_WORKERS:="$((WEBLATE_WORKERS <= 3 ? 2 : WEBLATE_WORKERS / 2))"}"
 
     export CELERY_MAIN_OPTIONS