`Connection#structure_dump` is no longer used

Origin: #9518

As of ccc6910c we use `mysqldump` to create the `structure.sql`.
The old `#structure_dump` code is still in AR but never used.

I removed all relevant parts from the code-base.

Even though this is just a cleanup I added a CHANGELOG entry. There could be gem authors or app owners that used `#structure_dump`, which will no longer be available.

I've also added a commit to issue a warning when `mysqldump` is not in the PATH. The rake task used to fail silently and left an empty `structure.sql`, which is not very helpful.