cassini-bsp: Corstone-1000 builds skip components provided by firmware

bitbake complains when multiple recipes deploy the same files. This
occurs for Corstone-1000 builds because:
- some recipes are built for both default and firmware configs
- some firmware binaries are then deployed into the default build

Fix the error by removing these recipes from the default build
- external-system
- trusted-firmware-m

Signed-off-by: Adam Johnston <adam.johnston@arm.com>