modules: mbedtls: remove MBEDTLS_ECP_MAX_BITS from mbedTLS configs

According to mbedTLS' Changelog "Mbed TLS 3.0.0 branch released
2021-07-07" -> "Removals":

   MBEDTLS_ECP_MAX_BITS is no longer a configuration option because it
   is now determined automatically based on supported curves.

Hence remove MBEDTLS_ECP_MAX_BITS from configuration files to fix build
issues with Zephyr when there is unfortunate order of include
statements.

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