x86, um: get rid of uml signal.h

the only theoretical reason for it these days is ppc; aside of uml/ppc
being dead, do_signal() would be happier in arch/powerpc/kernel/signal.h
anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>