tty/serial: cleanup after ioc*_serial driver removal

tty/serial: cleanup after ioc*_serial driver removal

Commit 55e1a24cf59d ("tty/serial: remove the ioc3_serial driver") and
commit 3d42f18c5c4a ("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>