Revert "util: Add "/bin" to PATH when spawning helpers"

This reverts commit 66ab5e1ad8ac682c9225446747885184b7bf41b5.

After Andrey's fixes that create mount points before dropping
old mounts and going to pivot_root, this patch is not needed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>