um: Drop own definition of PTRACE_SYSEMU/_SINGLESTEP

32bit UML used to define PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP
own its own because many years ago not all libcs had these request codes
in their UAPI.
These days PTRACE_SYSEMU/_SINGLESTEP is well known and part of glibc
and our own define becomes problematic.

With change c48831d0eebf ("linux/x86: sync sys/ptrace.h with Linux 4.14
[BZ #22433]") glibc turned PTRACE_SYSEMU/_SINGLESTEP into a enum and
UML failed to build.

Let's drop our define and rely on the fact that every libc has
PTRACE_SYSEMU/_SINGLESTEP.

Cc: <stable@vger.kernel.org>
Cc: Ritesh Raj Sarraf <rrs@researchut.com>
Reported-and-tested-by: Ritesh Raj Sarraf <rrs@researchut.com>
Signed-off-by: Richard Weinberger <richard@nod.at>