reforis: drop python3-simplejson dependency

It seems that we are not using simplejson in reForis anymore and we are
using Python's built-in json.

Reference:
https://github.com/openwrt/packages/pull/19182

web/reforis/reforis/Makefile

@@ -51,7 +51,6 @@ define Package/reforis
     +python3-flup \
     +python3-light \
     +python3-paho-mqtt \
-    +python3-simplejson \
     +turris-maintain
   VARIANT:=python3
 endef