fix: remove unused queue

Removed because causes unnecessary confusion. A queue called
"background" never existed it was commited by mistake while developing a
feature but that feature ended up not using it.

reference commit: e6adf91029
reference PR: https://github.com/frappe/erpnext/pull/26701

frappe/utils/background_jobs.py

@@ -20,7 +20,6 @@ from frappe.utils.commands import log
 
 default_timeout = 300
 queue_timeout = {
-	'background': 2500,
 	'long': 1500,
 	'default': 300,
 	'short': 300