Make PREVENT_STUCK_MODIFIERS the default (#3107)

* Remove chording as it is not documented, not used, and needs work.

* Make Leader Key an optional feature.

* Switch from `PREVENT_STUCK_MODIFIERS` to `STRICT_LAYER_RELEASE`

* Remove `#define PREVENT_STUCK_MODIFIERS` from keymaps.

keyboards/alf/x2/config.h

@@ -43,9 +43,6 @@
     keyboard_report->mods == (MOD_BIT(KC_LSHIFT) | MOD_BIT(KC_RSHIFT)) \
 )
 
-/* prevent stuck modifiers */
-#define PREVENT_STUCK_MODIFIERS
-
 #define RGB_DI_PIN E2
 #ifdef RGB_DI_PIN
 #define RGBLIGHT_ANIMATIONS
@@ -55,4 +52,4 @@
 #define RGBLIGHT_VAL_STEP 8
 #endif
 
-#endif
+#endif