cipher: remove unnecessary precondition

Already checked by parameter validation macros
diff --git a/library/cipher.c b/library/cipher.c
index 2636c09..2465536 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -283,12 +283,6 @@
     if( ctx->cipher_info == NULL )
         return( MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA );
 
-    if( operation != MBEDTLS_DECRYPT &&
-        operation != MBEDTLS_ENCRYPT )
-    {
-        return( MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA );
-    }
-
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     if( ctx->psa_enabled == 1 )
     {