nubus: Kill nubus_proc_detach_device()

Commit 59d8053f ("proc: Move non-public
stuff from linux/proc_fs.h to fs/proc/internal.h") broke Apple NuBus
support:

drivers/nubus/proc.c: In function ‘nubus_proc_detach_device’:
drivers/nubus/proc.c:156: error: dereferencing pointer to incomplete type
drivers/nubus/proc.c:158: error: dereferencing pointer to incomplete type

Fortunately nubus_proc_detach_device() is unused, and appears to have never
been used, so just remove it.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>