Remove deprecated `isLeftHand` (#25897)

Refactor deprecated isLeftHand

quantum/split_common/split_util.c

@@ -56,8 +56,6 @@
 
 static uint8_t connection_errors = 0;
 
-volatile bool isLeftHand = true;
-
 static struct {
     bool master;
     bool left;
@@ -203,8 +201,6 @@ void split_pre_init(void) {
     split_config.master = is_keyboard_master_impl();
     split_config.left   = is_keyboard_left_impl();
 
-    isLeftHand = is_keyboard_left(); // TODO: Remove isLeftHand
-
 #if defined(RGBLIGHT_ENABLE) && defined(RGBLED_SPLIT)
     uint8_t num_rgb_leds_split[2] = RGBLED_SPLIT;
     if (is_keyboard_left()) {

quantum/split_common/split_util.h

@@ -5,8 +5,6 @@
 
 #include "matrix.h"
 
-extern volatile bool isLeftHand;
-
 void split_pre_init(void);
 void split_post_init(void);