crypto-util: simplify openssl_extract_public_key()

crypto-util: simplify openssl_extract_public_key()

Drop memstream and i2d_PUBKEY_fp(). We can simply use i2d_PUBKEY()
which automatically allocates the necessary buffer for us.

Note that dropping the secure erase (erase_and_freep()) in favor of
OPENSSL_free() is intentional and safe, as the buffer only holds
public key material which does not need to be securely wiped.