Remove `QMK_SUBPROJECT` definitions in makefile (#15958)

build_keyboard.mk

@@ -425,8 +425,7 @@ OUTPUTS := $(KEYMAP_OUTPUT) $(KEYBOARD_OUTPUT)
 $(KEYMAP_OUTPUT)_SRC := $(SRC)
 $(KEYMAP_OUTPUT)_DEFS := $(OPT_DEFS) \
 -DQMK_KEYBOARD=\"$(KEYBOARD)\" -DQMK_KEYBOARD_H=\"$(QMK_KEYBOARD_H)\" \
--DQMK_KEYMAP=\"$(KEYMAP)\" -DQMK_KEYMAP_H=\"$(KEYMAP).h\" -DQMK_KEYMAP_CONFIG_H=\"$(KEYMAP_PATH)/config.h\" \
--DQMK_SUBPROJECT -DQMK_SUBPROJECT_H -DQMK_SUBPROJECT_CONFIG_H
+-DQMK_KEYMAP=\"$(KEYMAP)\" -DQMK_KEYMAP_H=\"$(KEYMAP).h\" -DQMK_KEYMAP_CONFIG_H=\"$(KEYMAP_PATH)/config.h\"
 $(KEYMAP_OUTPUT)_INC :=  $(VPATH) $(EXTRAINCDIRS)
 $(KEYMAP_OUTPUT)_CONFIG := $(CONFIG_H)
 $(KEYBOARD_OUTPUT)_SRC := $(PLATFORM_SRC)