stm32/boards/NUCLEO_F413ZH: Remove STARTUP_FILE, it's defined globally.

The Makefile now defines this variable to the correct value (but it can
still be overridden by a board if necessary).

ports/stm32/boards/NUCLEO_F413ZH/mpconfigboard.mk

@@ -1,6 +1,5 @@
 MCU_SERIES = f4
 CMSIS_MCU = STM32F413xx
-STARTUP_FILE = boards/startup_stm32f413xx.o
 AF_FILE = boards/stm32f413_af.csv
 LD_FILES = boards/stm32f413xh.ld boards/common_ifs.ld
 TEXT0_ADDR = 0x08000000