docs: remove some replace macros like |struct foo|

There are three files with replace macros for structs,
mapping them into Sphinx 2.x C domain references.

Well, this is broken on Sphinx 3.x. Also, for Sphinx 2.x,
the automarkup macro should be able to take care of them.

So, let's just drop those.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>