remove some anachronisms from the docs
When storing an object, empty flexible fields are removed from the xxx_attributes DB table.
Cancel this change. Will find another solution.

Revert "When storing an object, empty flexible fields are removed from the xxx_attributes DB table."

This reverts commit c317f573e30b57bd1ef358ee443c750dba939363.
Merge branch 'master' of https://github.com/brilnius/beets