Remove check for RSA encryption allowing X9.31 padding.

X9.31 is a Signature Standard, and should not apply to encryption.

rsa_ossl_public_encrypt() does not allow this padding mode.
The openssl rsautil command line tool already failed if the
-x931 option was used with -encrypt

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/24938)

providers/implementations/asymciphers/rsa_enc.c

@@ -51,7 +51,6 @@ static OSSL_ITEM padding_item[] = {
     { RSA_NO_PADDING,           OSSL_PKEY_RSA_PAD_MODE_NONE },
     { RSA_PKCS1_OAEP_PADDING,   OSSL_PKEY_RSA_PAD_MODE_OAEP }, /* Correct spelling first */
     { RSA_PKCS1_OAEP_PADDING,   "oeap"   },
-    { RSA_X931_PADDING,         OSSL_PKEY_RSA_PAD_MODE_X931 },
     { 0,                        NULL     }
 };