SQL performance: Removed filter to hide old articles

As discussed on
https://github.com/marienfressinaud/FreshRSS/issues/493#issuecomment-56266415
For performance, but this is also a good simplification.
Will help with PostgreSQL
https://github.com/marienfressinaud/FreshRSS/issues/416 and SQLite too
https://github.com/marienfressinaud/FreshRSS/issues/100 because the main
query becomes simpler.

Me may need to introduce another system to hide old articles, if this is
a problem for some users.
Also, the feature showing "empty feeds" in another colour may need to be
adapted.