Remove pid location for sidekiq, use default one (/discourse/tmp/pids)

config/sidekiq.yml

-:concurrency: ${SIDEKIQ_CONFIG_CONCURRENCY}
-:pidfile: /var/run/sidekiq.pid
 production:
+  :concurrency: ${SIDEKIQ_CONFIG_CONCURRENCY}
   :queues:
     - [critical, 8]
     - [default, 4]
     - [low, 2]
-    - [ultra_low]
\ No newline at end of file
+    - [ultra_low]  
\ No newline at end of file