makes: remove local IMXRT* defines from CFLAGS

The reason for this is a recent change to phoenix-rtos-build which
provides __TARGET and __CPU globally. So local definition of IMXRT*
became redundant.

JIRA: RTOS-519

devices/flash-flexspi/Makefile

@@ -9,13 +9,10 @@
 FLEXSPI_OBJS := fspi/fspi.o
 
 ifneq (, $(findstring imxrt117x, $(TARGET_SUBFAMILY)))
-	CFLAGS += -DIMXRT117X
 	FLEXSPI_OBJS += flashdrv.o nor/nor.o nor/nor_mx.o
 else ifneq (, $(findstring imxrt106x, $(TARGET_SUBFAMILY)))
-	CFLAGS += -DIMXRT106X
 	FLEXSPI_OBJS += flashdrv.o nor/nor.o nor/nor_mx.o
 else ifneq (, $(findstring imxrt105x, $(TARGET_SUBFAMILY)))
-	CFLAGS += -DIMXRT105X
 	FLEXSPI_OBJS += flashdrv.o nor/nor.o hyperbus/hyper.o
 endif