tests/crypto/mbedtls: Disable CONFIG_FORCE_NO_ASSERT

The root cause for this issue was found in
https://github.com/zephyrproject-rtos/zephyr/issues/6470


so this kconfig isn't needed anymore.

This is a partial reversion of 6eef2f14.  The actual feature is left
in place, as it's plausibly useful in other contexts (otherwise
assertions enabled by sanitycheck can't be disabled by the app as
they're passed in via CFLAGS).

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>