core/kernel_defines: drop CONST definition

This is not used anywhere in RIOT, but causes conflicts with external
library code.

So let's just get rid of this.

core/lib/include/kernel_defines.h

@@ -84,18 +84,6 @@ extern "C" {
 #define NORETURN
 #endif
 
-/**
- * @def CONST
- * @brief A function declared as *CONST* is #PURE and also not allowed to
- *        examine global memory. I.e. a *CONST* function cannot even
- *        dereference a pointer parameter.
- */
-#ifdef __GNUC__
-#define CONST  __attribute__((const))
-#else
-#define CONST
-#endif
-
 /**
  * @def PURE
  * @brief The function has no effects except the return value and its return