samples: crypto: remove default mbedTLS options
samples: crypto: remove default mbedTLS options

Whenever MBEDTLS_BUILTIN is selected then
CONFIG_MBEDTLS_CFG_FILE="config-tls-generic.h" is set as a default value.

Remove explicit configuration from prj_mtls_shim.conf.

Signed-off-by: Marcin Niestroj <63506c06cfbc47ace147db1702f6e751f5ac2132@emb.dev>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr