feerate: remove duplicate method

is dupe of `bitcoin_tx_core_weight`

bitcoin/feerate.c

@@ -39,12 +39,3 @@ const char *feerate_style_name(enum feerate_style style)
 	}
 	abort();
 }
-
-size_t common_weight(size_t num_inputs, size_t num_outputs)
-{
-	/*(nVersion + num inputs + num outputs + locktime) * 4
-	 *         + SegWit marker + SegWit flag */
-	return (4 + varint_size(num_inputs) +
-		varint_size(num_outputs) + 4) * 4
-		+ 1 + 1;
-}