Remove unused macro

We have now fully replaced the usage of this private macro, remove it.

src/macros.rs

@@ -1,18 +1,5 @@
 // SPDX-License-Identifier: CC0-1.0
 
-/// Combines the given `$thing` field of two types by setting `self.thing` to be
-/// the value in `other.thing` iff `self.thing` is currently empty.
-///
-/// If `self.thing` already contains a value then this macro does nothing.
-#[allow(unused_macros)]
-macro_rules! combine {
-    ($thing:ident, $slf:ident, $other:ident) => {
-        if let (&None, Some($thing)) = (&$slf.$thing, $other.$thing) {
-            $slf.$thing = Some($thing);
-        }
-    };
-}
-
 /// Combines two `Option<Foo>` fields.
 ///
 /// Sets `self.thing` to be `Some(other.thing)` iff `self.thing` is `None`.