Merge #19846: build: enable unused member function diagnostic

819d03b932 refactor: took out unused member functions (Zero)
ed69213c2b build: enable unused member function diagnostic (Zero)

Pull request description:

  This PR enables the `-Wunused-member-function` compiler diagnostic, as discussed in #19702.

  > **Notice**: The `unused-member-function` diagnostic is only available on clang. Therefore, clang should be used to test this PR.

  - [x] Include the `-Wunused-member-function`diagnostic in `./configure.ac`. (ed69213c2b)
  - [x] Resolve the reported warnings. (819d03b932)

  Currently, enabling this flag no longer reports the following warnings:

  > **Note**: output from `make 2>&1 | grep "warning: unused member function" | sort | uniq -c`

  ```
  1 index/blockfilterindex.cpp:54:5: warning: unused member function 'DBHeightKey' [-Wunused-member-function]
  2 script/bitcoinconsensus.cpp:50:9: warning: unused member function 'GetType' [-Wunused-member-function]
  1 test/util_tests.cpp:1975:14: warning: unused member function 'operator=' [-Wunused-member-function]
  ```

  All tests have passed locally (from `make check` & `src/test/test_bitcoin`).

  This PR closes #19702.

ACKs for top commit:
  practicalswift:
    ACK 819d03b932 - patch still looks correct :)
  MarcoFalke:
    ACK 819d03b932
  pox:
    Tested ACK 819d03b932 with clang after `make clean`. No unused member function warnings.
  theStack:
    tested ACK 819d03b932

Tree-SHA512: 5fdfbbb02b3dc618a90a874a5caa5e01e596fc1d14a209e75a6981f01b253f9bca0cfac8fdd758dd7151986609fb76571c3745124a29cfd4f8cbb8d82a07272e

src/test/util_tests.cpp

@@ -2014,12 +2014,6 @@ struct Tracker
         copies = t.copies + 1;
         return *this;
     }
-    Tracker& operator=(Tracker&& t) noexcept
-    {
-        origin = t.origin;
-        copies = t.copies;
-        return *this;
-    }
 };
 
 }