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/s60_x/keymaps/bluebear/config.h

@@ -105,7 +105,4 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 // Space Cadet Rollover - if set, allows to tap opposite shift key to cancel erroneous press
 #define DISABLE_SPACE_CADET_ROLLOVER
 
-// Prevent stuck modifiers
-#define PREVENT_STUCK_MODIFIERS
-
 #endif

keyboards/s60_x/rgb/config.h

@@ -17,9 +17,6 @@
 /* Locking resynchronize hack */
 #define LOCKING_RESYNC_ENABLE
 
-/* prevent stuck modifiers */
-#define PREVENT_STUCK_MODIFIERS
-
 #define RGB_DI_PIN F6
 #ifdef RGB_DI_PIN
 #define RGBLIGHT_ANIMATIONS
@@ -29,4 +26,4 @@
 #define RGBLIGHT_VAL_STEP 8
 #endif
 
-#endif
+#endif