Move KEY_INDEX to the hardware plugin

To allow the hardware plugin to use a more efficient way of representing the
index (if need be), and to be able to turn it into a constexpr, move it to the
hardware plugin.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope.h

@@ -70,16 +70,6 @@ static_assert(KALEIDOSCOPE_REQUIRED_API_VERSION == KALEIDOSCOPE_API_VERSION,
 
 const uint8_t KEYMAP_SIZE DEPRECATED(KEYMAP_SIZE) = 0;
 
-/* To be used by the hardware implementations, `KEY_INDEX` tells us the index of
- * a key, from which we can figure out the row and column as needed. The index
- * starts at one, so that plugins that work with a list of key indexes can use
- * zero as a sentinel. This is important, because when we initialize arrays with
- * fewer elements than the declared array size, the remaining elements will be
- * zero. We can use this to avoid having to explicitly add a sentinel in
- * user-facing code.
- */
-#define KEY_INDEX(row, col) (row * COLS + col + 1)
-
 namespace kaleidoscope {
 
 class Kaleidoscope_ {