NativeThreadLinux: remove translation of some Linux signals to gdb signals.

I copied this originally based on what debugserver was doing.  This appears to
be incorrect and unncessary for Linux.  The LinuxSignals on the lldb side
don't look for these and therefore they get handled incorrectly.

Leaving the hook in place since I think darwin will continue to need to
translate those signal numbers.

llvm-svn: 216564