Only silence ENOENT in helpers.unlink

This is what it is documented as doing and what makes sense in this
case.

Signed-off-by: Nathan Bird <ecthellion@gmail.com>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>