docs: kdoc: remove the "struct_actual" machinery

docs: kdoc: remove the "struct_actual" machinery

The code goes out of its way to create a special list of parameters in
entry.struct_actual that is just like entry.parameterlist, but with extra
junk.  The only use of that information, in check_sections(), promptly
strips all the extra junk back out.  Drop all that extra work and just use
parameterlist.

No output changes.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>