mk/git.sh: remove tree_depend variable

this was used alongside the xgcc linking, so that coreboot
trees could specify that another tree was to be downloaded.

since this variable will no longer be used, it should be
removed, to avoid dead code bloat.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/git.sh

@@ -9,8 +9,6 @@ tmpgit="$xbmkpwd/tmp/gitclone"
 
 fetch_targets()
 {
-	[ -n "$tree_depend" ] && [ "$tree_depend" != "$tree" ] && \
-		x_ ./mk -f "$project" "$tree_depend"
 	e "src/$project/$tree" d && return 0
 
 	printf "Creating %s tree %s\n" "$project" "$tree"
@@ -21,7 +19,7 @@ fetch_targets()
 
 fetch_project()
 {
-	eval "`setvars "" xtree tree_depend`"
+	eval "`setvars "" xtree`"
 	eval "`setcfg "config/git/$project/pkg.cfg"`"
 
 	chkvars url