fix: remove 'u' before string characters as redundant in Python 3

Author: casperr0

mamonsu/lib/zbx_template.py

@@ -6,7 +6,7 @@
 
 class ZbxTemplate(object):
     plg_type = 'all'
-    mainTemplate = u"""<?xml version="1.0" encoding="UTF-8"?>
+    mainTemplate = """<?xml version="1.0" encoding="UTF-8"?>
 <zabbix_export>
 <version>2.0</version>
     <groups>