[dataimport] drop massive store's flush_metadata method
This method handle a temporary table which sounds useless, and even buggy in
some cases (metadata for a single entity types will be only flushed once even if
flush is called several time). Instead, simply call the method-doing-the job
after entities have been inserted (and make it private along the way).
3.23 (UNRELEASED)
=================
Backwards-incompatible changes
------------------------------
* Generative tests *à la logilab-common* are not supported anymore in
`CubicWebTC`. It is advised to use the subtests_ API (available on
`CubicWebTC` either from the standard library as of Python 3.4 or through
unittest2 package otherwise).
.. _subtests: https://docs.python.org/3/library/unittest.html#distinguishing-test-iterations-using-subtests