`Connection#structure_dump` is no longer used. #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.
rake db:structure:dump warns when `mysqldump` is not in PATH.

Closes #9518.

The rake task used to fail silently and left an empty `structure.sql`.
It's confusing for users to get to the root of the problem.
The warning message tells them where to look.
Merge pull request #9538 from senny/remove_structure_dump

`Connection#structure_dump` is no longer used