sound: Retire OSS

Since no complaints have been raised after disabling the build of OSS
(Open Sound System) by the commit 31cbee6a5611 ("sound: Disable the
build of OSS drivers"), let's finally drop the whole code and
documentation.

Some glue codes are still left intact since sound/oss/dmasound stuff
remains -- which is an independent implementation solely for m68k, and
it's not covered by ALSA yet.

Also, a couple of API header files (linux/sound.h and
linux/soundcard.h) are kept remaining as well, since the OSS API
itself is still supported by ALSA OSS emulation, and applications can
refer to these.

Where we're at it, some help texts in the top-level Kconfig are
adjusted, too (who still needs to specify I/O port in kbuild
nowadays?).

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
