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/signal.c

@@ -19,7 +19,6 @@
 #include "qemu/osdep.h"
 
 #include "qemu.h"
-#include "target_signal.h"
 
 void signal_init(void)
 {