[nrf noup] boot: zephyr: Remove prj_minimal.conf

The file is now deprecated:
- it does not provide the minimal configuration
- build image cannot even fit into flash memory
- smp_svr_mini_boot sample is a new reference for minimal config

Ref: NCSIDB-1056

Signed-off-by: Adam Szczygieł <adam.szczygiel@nordicsemi.no>

Author: adsz-nordic