docs: Remove make headers_check from checklist

Remove the make headers_check step from submit-checklist.rst as this is
no longer functional.

Signed-off-by: Milan Lakhani <milan.lakhani@codethink.co.uk>
Link: https://lore.kernel.org/r/1610458861-2832-1-git-send-email-milan.lakhani@codethink.co.uk
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed