kernel.h: Remove unused FIELD_SIZEOF()

Now that all callers of FIELD_SIZEOF() have been converted to
sizeof_field(), remove the unused prior macro.
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/kernel.h

@@ -79,15 +79,6 @@
  */
 #define round_down(x, y) ((x) & ~__round_mask(x, y))
 
-/**
- * FIELD_SIZEOF - get the size of a struct's field
- * @t: the target struct
- * @f: the target struct's field
- * Return: the size of @f in the struct definition without having a
- * declared instance of @t.
- */
-#define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
-
 #define typeof_member(T, m)	typeof(((T*)0)->m)
 
 #define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP