MAINTAINERS: Remove the ipx network layer info

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
remove the ipx network layer info from MAINTAINERS.
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>