zephyr: 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, like:

  /__w/zephyr/zephyr/modules/mbedtls/zephyr_init.c
  In file included from /__w/zephyr/modules/crypto/mbedtls/include/mbedtls/ssl.h:30,
                   from /__w/zephyr/modules/crypto/mbedtls/include/mbedtls/debug.h:27,
                   from /__w/zephyr/zephyr/modules/mbedtls/zephyr_init.c:16:
  /__w/zephyr/modules/crypto/mbedtls/include/mbedtls/ecp.h:314: error: "MBEDTLS_ECP_MAX_BITS" redefined [-Werror]
    314 | #define MBEDTLS_ECP_MAX_BITS 1
        |
  In file included from /__w/zephyr/bootloader/mcuboot/boot/zephyr/include/mcuboot-mbedtls-cfg.h:25,
                   from /__w/zephyr/modules/crypto/mbedtls/include/mbedtls/build_info.h:59,
                   from /__w/zephyr/modules/crypto/mbedtls/include/mbedtls/debug.h:25,
                   from /__w/zephyr/zephyr/modules/mbedtls/zephyr_init.c:16:
  /__w/zephyr/bootloader/mcuboot/boot/zephyr/include/config-rsa.h:72: note: this is the location of the previous definition
     72 | #define MBEDTLS_ECP_MAX_BITS             2048
        |
  cc1: all warnings being treated as errors

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