modules: mbedtls: remove default-enabling of hash algorithms

Do not enable hash algorithms except SHA-256 by default.
This unnecessarily inflates the final code size even if not all the
enabled hash algorithms are actually used.

SHA-256 is (for now) kept enabled by default because many configurations
across the code base assume that there is some hash algorithm
available without needing to enable it.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>