Dropping a co-convener from the list?
Conveners change. After five years, surely change is expected. Dropping one of your co-conveners is a little suspect though, it suggests an individual is at fault. I recommend dropping several co-conveners and having a more radical re-shuffle.

Because the title and proposal are same as previous years, is it ethical to drop one who was already a co-convener of the same session since last 5 years?

Assuming you have no agreement, then, yes, it is ethical to drop and add co-conveners. Journals and conferences typically work this way.

How to inform the one that you won't be in the list this year?

That's been partly answered elsewhere on this site and is partly dependent on your individual circumstances.