remove koa-session middleware dependency

[uptownhr]

I think the koa-session dependency is not warranted and should be removed. This prevents from other session middle-ware being used to work for no reason.

I'm currently working with koa-generic-session as it's documented in koa-passport - i'm sure koa-session works too. But I wanted a db store for my sessions. Since both koa-sessio nand koa-generic-session both uses ctx.req.session, I think koa-flash should check to see that ctx.req.session is available as it's check.