Removed all instances of crash_prog()

crash_prog was introduced to force-crash a userspace program
since _exit may not yet be implemented. However, the new versions
of OS161 already have the exact same logic as crash_prog in stdlib/exit.

userland/include/err.h

@@ -55,6 +55,4 @@ void vwarnx(const char *fmt, __va_list);
 __DEAD void verr(int exitcode, const char *fmt, __va_list);
 __DEAD void verrx(int exitcode, const char *fmt, __va_list);
 
-void crash_prog(void);
-
 #endif /* _ERR_H_ */