[REM] asynchronous database creation method

It's broken (though easy to fix) and not very useful, if third parties want to
create databases asynchronously they can handle the asynchronicity on the
client-side (an HTTP request is easy to make asynchronously after all) and
call the synchronous `create_database()`.

fixes #1137, after a fashion
Merge pull request #1169 from xmo-odoo/8.0-remove-async-db-create-xmo

Remove asynchronous XML-RPC database creation function