gdbsupport: remove #ifndef REALTIME_LO in signals.cc

REALTIME_LO was only possibly previously defined in config/nm-nto.h,
which is now removed.  So we can remove the #ifndef protecting against a
redefinition of REALTIME_LO in gdbsupport/signals.cc.

Change-Id: I021b9518ceaa6223bd480ff1e07e9a978b0b241e
Approved-by: Kevin Buettner <kevinb@redhat.com>