net: Remove net/ipx.h and uapi/linux/ipx.h header files

commit <47595e32> ("<MAINTAINERS: Mark some staging directories>")
indicated the ipx network layer as obsolete in Jan 2018,
updated in the MAINTAINERS file

now, after being exposed for 3 years to refactoring, so to
delete uapi/linux/ipx.h and net/ipx.h header files for good.
additionally, there is no module that depends on ipx.h except
a broken staging driver(r8188eu)

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>