remove dbapi_proxy module

remove dbapi_proxy module

This module was not documented nor part of any test suite,
and it's unlikely it was working correctly.   It's not likely
that this module was ever used after the first year or so
of SQLAlchemy, and it's stayed around because it is so
obscure that I never remembered to remove it.

Change-Id: I0ed9030438982e935add87c51abbfff50e7382be
References: #7257