Fix dropping of indexed columns in sqlite/sa08

Version 0.8 of SQLAlchemy added support of indexes
on expressions in addition to plain table columns,
which changed the way indexes are created.

This broke support of dropping columns of composite
indexes for SQLite: due to limitations of ALTER in
SQLite every time a column is dropped, we recreate
the whole table without the given column; if a
column is a part of a composite index, we change the
index definition to omit that column and then indexes
are recreated too.

SQLAlchemy versions starting from 0.8 no more pay
attention to 'columns' attribute of Index instances
when generating DDL for indexes, so when one of columns
of a composite index is dropped, we try to create a
new index on the column that doesn't exist anymore,
which of course fails.

Closes-Bug: #1241038

Change-Id: I777b8ce36e36f49bfb0889908811a063cf1a527b