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.ukSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/submit-checklist.rst

@@ -89,30 +89,28 @@ and elsewhere regarding submitting Linux kernel patches.
     Patches that change userspace interfaces should be CCed to
     linux-api@vger.kernel.org.
 
-19) Check that it all passes ``make headers_check``.
-
-20) Has been checked with injection of at least slab and page-allocation
+19) Has been checked with injection of at least slab and page-allocation
     failures.  See ``Documentation/fault-injection/``.
 
     If the new code is substantial, addition of subsystem-specific fault
     injection might be appropriate.
 
-21) Newly-added code has been compiled with ``gcc -W`` (use
+20) Newly-added code has been compiled with ``gcc -W`` (use
     ``make EXTRA_CFLAGS=-W``).  This will generate lots of noise, but is good
     for finding bugs like "warning: comparison between signed and unsigned".
 
-22) Tested after it has been merged into the -mm patchset to make sure
+21) Tested after it has been merged into the -mm patchset to make sure
     that it still works with all of the other queued patches and various
     changes in the VM, VFS, and other subsystems.
 
-23) All memory barriers {e.g., ``barrier()``, ``rmb()``, ``wmb()``} need a
+22) All memory barriers {e.g., ``barrier()``, ``rmb()``, ``wmb()``} need a
     comment in the source code that explains the logic of what they are doing
     and why.
 
-24) If any ioctl's are added by the patch, then also update
+23) If any ioctl's are added by the patch, then also update
     ``Documentation/userspace-api/ioctl/ioctl-number.rst``.
 
-25) If your modified source code depends on or uses any of the kernel
+24) If your modified source code depends on or uses any of the kernel
     APIs or features that are related to the following ``Kconfig`` symbols,
     then test multiple builds with the related ``Kconfig`` symbols disabled
     and/or ``=m`` (if that option is available) [not all of these at the