Remove legacy keycodes, part 4 (#18683)

* `KC_PGDOWN` -> `KC_PGDN`

* `KC_PSCREEN` -> `KC_PSCR`

* `KC_SCOLON` -> `KC_SCLN`

* `KC_BSLASH` -> `KC_BSLS`

* `KC_BSPACE` -> `KC_BSPC`

quantum/keycode_legacy.h

@@ -4,12 +4,6 @@
 
 // These keycode names have been deprecated
 
-#define KC_BSPACE         KC_BACKSPACE
-#define KC_BSLASH         KC_BACKSLASH
-#define KC_SCOLON         KC_SEMICOLON
-#define KC_PSCREEN        KC_PRINT_SCREEN
-#define KC_PGDOWN         KC_PAGE_DOWN
-
 #define KC_LCTRL          KC_LEFT_CTRL
 #define KC_LSHIFT         KC_LEFT_SHIFT
 #define KC_RCTRL          KC_RIGHT_CTRL