Unset trap before dracut ramdisk build script exits

A similar change was needed in the normal ramdisk element as part
of ae928057bd2fc2205c91bf052d5503de64f9ea69 to avoid running
cleanup immediately after the ramdisk build completes.

Change-Id: Ia96e2d8011b88ed96cc6727914c5a5d2dea59757
Merge "Unset trap before dracut ramdisk build script exits"