Remove incorrect todo

We are not going to depend on the `internals` crate because we only need
this error macro - we can keep it as is.

Author: tcharding

src/error.rs

@@ -1,8 +1,6 @@
 // Written by the Rust Bitcoin developers.
 // SPDX-License-Identifier: CC0-1.0
 
-// TODO: We can get this from `bitcoin-internals` crate once that is released.
-
 //! # Error
 //!
 //! Error handling macros and helpers.