[Core] Remove matrix_is_modified() and debounce_is_active() (#15349)

quantum/matrix.h

@@ -46,8 +46,6 @@ void matrix_setup(void);
 void matrix_init(void);
 /* scan all key states on matrix */
 uint8_t matrix_scan(void);
-/* whether modified from previous scan. used after matrix_scan. */
-bool matrix_is_modified(void) __attribute__((deprecated));
 /* whether a switch is on */
 bool matrix_is_on(uint8_t row, uint8_t col);
 /* matrix state on row */