runtime: remove unused var

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12249043

src/pkg/runtime/traceback_arm.c

@@ -9,8 +9,6 @@
 
 void runtime·sigpanic(void);
 
-static String unknown = { (uint8*)"?", 1 };
-
 int32
 runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip, uintptr *pcbuf, int32 max, void (*callback)(Stkframe*, void*), void *v, bool printall)
 {

src/pkg/runtime/traceback_x86.c

@@ -17,8 +17,6 @@ void runtime·sigpanic(void);
 // This code is also used for the 386 tracebacks.
 // Use uintptr for an appropriate word-sized integer.
 
-static String unknown = { (uint8*)"?", 1 };
-
 // Generic traceback.  Handles runtime stack prints (pcbuf == nil),
 // the runtime.Callers function (pcbuf != nil), as well as the garbage
 // collector (callback != nil).  A little clunky to merge these, but avoids