docs: fs: api-summary.rst: get rid of kernel-doc include

docs: fs: api-summary.rst: get rid of kernel-doc include

The direct-io.c file used to have just two exported symbols:

	- dio_end_io()
	- __blockdev_direct_IO()

The first one was removed by changeset
c33fe275b530 ("fs: remove no longer used dio_end_io()")

And the last one is used on most places indirectly, via
the inline macro blockdev_direct_IO() provided by fs.h.
Yet, neither the macro or the function have kernel-doc
markups.

So, drop the inclusion of fs/direct-io.c at the docs.

Fixes: c33fe275b530 ("fs: remove no longer used dio_end_io()")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d0a9fffedca102633c168adaf157f34288a4ea67.1603791716.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>