internals: Remove attribution from all files

As we did for the `bitcoin` crate, remove attribution from all files in
the `internals` crate.

While we are at it add an SPDX line to the few files missing it, whether
this license nonsense is even needed is left as an argument for another
day.

Justification:

Currently we have a mishmash of attribution lines accompanying the SPDX
identifier. These lines are basically meaningless because:

    - The date is often wrong
    - The original author attributed is not the only contributor to a file
    - The term "rust bitcoin developers" is basically just noise

Just remove all the attribution lines and be done with it.

internals/src/error.rs

@@ -1,4 +1,3 @@
-// Written by the Rust Bitcoin developers.
 // SPDX-License-Identifier: CC0-1.0
 
 //! # Error

internals/src/hex/buf_encoder.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Implements a buffered encoder.
 //!
 //! The main type of this module is [`BufEncoder`] which provides buffered hex encoding. Such is

internals/src/hex/display.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Helpers for displaying bytes as hex strings.
 //!
 //! This module provides a trait for displaying things as hex as well as an implementation for

internals/src/hex/mod.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Helpers for encoding bytes as hex strings.
 
 pub mod buf_encoder;

internals/src/lib.rs

@@ -1,4 +1,3 @@
-// Written by the Rust Bitcoin developers.
 // SPDX-License-Identifier: CC0-1.0
 
 //! # Rust Bitcoin Internal

internals/src/macros.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Various macros used by the Rust Bitcoin ecosystem.
 //!