defaults: purge long unused LB_ROOT

seems to have been obsolete since all the way back at v1.0~a22-1.

history:
 - in 0d0de885e3 it was renamed from
   LIVE_ROOT to LH_ROOT, but also seems to have become completely
   obsolete at this point, and thus mistakenly not actually removed.
   before this it seems to have been used to hold the base directory of
   live-build from which paths were constructed, but then this use was
   removed making it redundant, but it remained in the code.
 - 83bc63f725 renamed to LB_ROOT.
 - a79a5bea10 dropped setting the variable
   only if not already set, in favour of always setting it depending upon
   LB_MODE. but still it remained unused.

Gbp-Dch: Short

functions/defaults.sh

@@ -279,17 +279,6 @@ Set_config_defaults ()
 	# Setting tasksel
 	LB_TASKSEL="${LB_TASKSEL:-apt}"
 
-	# Setting root directory
-	case "${LB_MODE}" in
-		progress-linux)
-			LB_ROOT="${LB_ROOT:-progress-linux}"
-			;;
-
-		*)
-			LB_ROOT="${LB_ROOT:-${LB_MODE}-live}"
-			;;
-	esac
-
 	# Setting live build options
 	if [ -z "${_COLOR}" ]; then
 		_COLOR="auto"