gdb+gdbserver/Linux: Remove USE_SIGTRAP_SIGINFO fallback

It's been over 9 years (since commit faf09f0119da) since Linux GDB and
GDBserver started relying on SIGTRAP si_code to tell whether a
breakpoint triggered, which is important for non-stop mode.  When that
then-new code was added, I had left the then-old code as fallback, in
case some architectured still needed it.  Given AFAIK there haven't
been complaints since, this commit finally removes the fallback code,
along with USE_SIGTRAP_SIGINFO.

Change-Id: I140a5333a9fe70e90dbd186aca1f081549b2e63d
3 files changed