Don't return anything from transfer precompile

The return value is not used and there is not point in returning the
input (which is the current behavior). Not returning anything also saves
some gas.

core/vm/celo_contracts.go

@@ -97,5 +97,5 @@ func (c *transfer) Run(input []byte, ctx *celoPrecompileContext) ([]byte, error)
 
 	ctx.Transfer(ctx.evm.StateDB, from, to, valueU256)
 
-	return input, nil
+	return nil, nil
 }