tty/serial: cleanup after ioc*_serial driver removal

Commit 9c860e4c ("tty/serial: remove the ioc3_serial driver") and
commit a017ef17

 ("tty/serial: remove the ioc4_serial driver") removed
the ioc{3,4}_serial driver, but missed some files.

Fortunately, ./scripts/get_maintainer.pl --self-test complains:

  warning: no file matches F: drivers/tty/serial/ioc?_serial.c

The driver is gone, so remove the header and maintainer
entry as well.

The serial.rst Documentation might be useful, so we keep it and update
the maintainer entry to the document's actual maintainers.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>