Use hash instead of which in expand-root

Also remove the variable since we established that it is already in the
$PATH

Signed-off-by: Andrew Garrett <andrew.garrett@getbraintree.com>

bootstrapvz/common/assets/init.d/expand-root

@@ -13,14 +13,14 @@
 prog=$(basename $0)
 logger="logger -t $prog"
 
-growpart=$(which growpart) || {
+hash growpart 2> /dev/null || {
 	$logger "growpart was not found on PATH. Unable to expand size."
 	exit 1
 }
 
 root_index="0"
 
-$growpart /dev/xvda $root_index || {
+growpart /dev/xvda $root_index || {
 	$logger "growpart failed. Unable to expand size."
 	exit 1
 }