Deprecate KEYMAP_SIZE too

This was only ever used for `Kaleidoscope.setup()`, and while the variant that
takes an argument is deprecated, and emits a warning already, we can do the same
for `KEYMAP_SIZE` too.

This does set the const to 0, so if used anywhere else than
`Kaleidoscope.setup()`, it will have undesired side-effects. But as far as I
saw, it was never used elsewhere, thus, this change should be safe.

Signed-off-by: Gergely Nagy <kaleidoscope@gergo.csillger.hu>

src/Kaleidoscope.h

@@ -33,7 +33,8 @@ extern HARDWARE_IMPLEMENTATION KeyboardHardware;
 #define VERSION "locally-built"
 #endif
 
-#define KEYMAP_SIZE (sizeof(keymaps) / ROWS / COLS / sizeof(Key))
+const uint8_t KEYMAP_SIZE
+__attribute__((deprecated("Kaleidoscope.setup() does not require KEYMAP_SIZE anymore."))) = 0;
 
 #define USE_PLUGINS(plugins...) Kaleidoscope.use(plugins)