Removed unused (and unimplemented) stackcheck() and stackoverflow() routines

These routines were stubs that did nothing and were not called from anywhere
so lack value in even providing a template or hook for a fuller implementation.
Also note that "stackcheck" conflicts with a predefined symbol in some runtime
libraries.  Should these be implemented in the future they should use a
maiko-specific name.

inc/commondefs.h

@@ -3,6 +3,4 @@
 void stab(void);
 void warn(const char *s);
 int error(const char *s);
-int stackcheck(void);
-void stackoverflow(void);
 #endif