wanrouter: delete now orphaned header content, files/drivers

The wanrouter support was identified earlier as unused for years,
and so the previous commit totally decoupled it from the kernel,
leaving the related wanrouter files present, but totally inert.

Here we take the final step in that cleanup, by doing a wholesale
removal of these files.  The two step process is used so that the
large deletion is decoupled from the git history of files that we
still care about.

The drivers deleted here all were dependent on the Kconfig setting
CONFIG_WAN_ROUTER_DRIVERS.

A stub wanrouter.h header (kernel & uapi) are left behind so that
drivers/isdn/i4l/isdn_x25iface.c continues to compile, and so that
we don't accidentally break userspace that expected these defines.

Cc: Joe Perches <joe@perches.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>