runtime: remove unneeded C header files

Change-Id: I239ae86cfebfece607dce39a96d9123cbacbee7d
Reviewed-on: https://go-review.googlesource.com/5562
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/cgocall.h

-// Copyright 2009 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-/*
- * Cgo interface.
- */
-
-void runtime·cgocall(void (*fn)(void*), void*);
-int32 runtime·cgocall_errno(void (*fn)(void*), void*);
-void runtime·cgocallback(void (*fn)(void), void*, uintptr);
-void *runtime·cmalloc(uintptr);
-void runtime·cfree(void*);

src/runtime/signal_unix.h

-// Copyright 2013 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-#define SIG_DFL ((void*)0)
-#define SIG_IGN ((void*)1)
-
-typedef void GoSighandler(int32, Siginfo*, void*, G*);
-void	runtime·setsig(int32, GoSighandler*, bool);
-GoSighandler* runtime·getsig(int32);
-
-void	runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp);
-void	runtime·raise(int32);
-