🐛 Fix MKS 'USB Flash MSC' environments (#22515)

ini/stm32f4.ini

 [env:mks_robin_nano_v3_usb_flash_drive_msc]
 platform          = ${common_stm32.platform}
 extends           = env:mks_robin_nano_v3_usb_flash_drive
-build_flags       = ${env:mks_robin_nano_v3_usb_flash_drive}
+build_flags       = ${env:mks_robin_nano_v3_usb_flash_drive.build_flags}
                     -DUSBD_USE_CDC_MSC
+build_unflags     = -DUSBD_USE_CDC
 
 #
 # This I2C1(PB8:I2C1_SCL PB9:I2C1_SDA) is used by MKS Monster8
 [env:mks_monster8_usb_flash_drive_msc]
 platform          = ${common_stm32.platform}
 extends           = env:mks_monster8_usb_flash_drive
-build_flags       = ${env:mks_monster8_usb_flash_drive}
+build_flags       = ${env:mks_monster8_usb_flash_drive.build_flags}
                     -DUSBD_USE_CDC_MSC
+build_unflags     = -DUSBD_USE_CDC