Reduce the weight of Bengali plugins

First, remove the word prediction database for bn-probhat. As it turns out, lomiri-keyboard will automatically use the database for the no-suffix variant automatically, so that saves up 1 copy of database. This means the Probhat layout will have to depend on Jatiya (traditional) layout, but since English's Dvorak layout do it too, I guess it's fine.

And since Probhat have to depends on Jatiya anyway, also replace the duplicated copy of symbol layout with the symlink. It's not much saving, but why not?

And finally, reduce the size of the word completion database itself by removing all but 1 of the source e-book. This reduces the size of the database down from ~62M to ~19M. It's still the largest database, but at least it's more resonable.

All these improvements should reduce the size of the rootfs to be under certain device's limit again.