Don't apply no-traverse to query.statement

Don't apply no-traverse to query.statement

Fixed long-standing issue in :class:`.Query` where a scalar subquery such
as produced by :meth:`.Query.exists`, :meth:`.Query.as_scalar` and other
derivations from :attr:`.Query.statement` would not correctly be adapted
when used in a new :class:`.Query` that required entity adaptation, such as
when the query were turned into a union, or a from_self(), etc. The change
removes the "no adaptation" annotation from the :func:`.select` object
produced by the :attr:`.Query.statement` accessor.

Change-Id: I554e0e909ac6ee785ec3b3b14aaec9d235aa28cf
Fixes: #4304