bsd-user: remove target_signal.h, it's unused

Remove the target_signal.h file. None of its contents are currently used and the
bsd-user fork doesn't use them (so this reduces the diffs there).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>

bsd-user/qemu.h

@@ -38,7 +38,6 @@ extern enum BSDType bsd_type;
 
 #include "syscall_defs.h"
 #include "target_syscall.h"
-#include "target_signal.h"
 #include "exec/gdbstub.h"
 
 #if defined(CONFIG_USE_NPTL)