Remove semi-internal chacha20_keystrem_block()

It's actually easy to implement chachapoly without it, so let's not clutter
the API (and avoid adding a burden to alt implementers).

include/mbedtls/chacha20.h

@@ -107,27 +107,6 @@ int mbedtls_chacha20_starts( mbedtls_chacha20_context* ctx,
                              const unsigned char nonce[12],
                              uint32_t counter );
 
-/**
- * \brief           Generates a block of keystream bytes for a specific counter value.
- *
- *                  This function uses the key and nonce previously set in
- *                  the context (via mbedtls_chacha20_setkey and
- *                  mbedtls_chacha20_starts), but ignores the previously
- *                  set counter and uses the counter given as the parameter to
- *                  this function.
- *
- * \param ctx       The ChaCha20 context. This context is not modified.
- * \param counter   The counter value to use.
- * \param keystream Buffer to where the generated keystream bytes are written.
- *
- * \return          MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if ctx or keystream are
- *                  NULL.
- *                  Otherwise, 0 is returned to indicate success.
- */
-int mbedtls_chacha20_keystream_block( const mbedtls_chacha20_context *ctx,
-                                      uint32_t counter,
-                                      unsigned char keystream[64] );
-
 /**
  * \brief           Encrypt or decrypt data.
  *