fix environment file in cubocore, gnome and lxqt overlays

overlays/cubocore/etc/environment

-#
-# This file is parsed by pam_env module
-#
-# Syntax: simple "KEY=VAL" pairs on separate lines
-#
-QT_QUICK_BACKEND=software

overlays/gnome/etc/environment

@@ -3,5 +3,4 @@
 #
 # Syntax: simple "KEY=VAL" pairs on separate lines
 #
-QT_QUICK_BACKEND=software
 QT_QPA_PLATFORMTHEME=qt5ct

overlays/lxqt/etc/environment

@@ -3,6 +3,4 @@
 #
 # Syntax: simple "KEY=VAL" pairs on separate lines
 #
-
-QT_QUICK_BACKEND=software
 QT_QPA_PLATFORMTHEME=qt5ct