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 <m.niestroj@emb.dev>