tests: crypto: mbedtls: remove default mbedTLS options

Whenever MBEDTLS_BUILTIN is selected then
CONFIG_MBEDTLS_CFG_FILE="config-tls-generic.h" is set as a default value.
It is even impossible to change it, as without CUSTOM_MBEDTLS_CFG_FILE it
is just a hidden (not configurable by user) Kconfig option.

Remove explicit configuration from prj.conf.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>