Fix minimal pool liquidity

[jak-pan]

I think this check was supposed to check the shares left after withdrawal to stop you from being locked out as there is similar check in XYK if I remember correctly.

However, this won't prevent it as you can just send shares away. This means we can end up in a state where there is enough shares in a pool but there is not enough shares in anybody's account to actually withdraw anything. I think the solution to this is to tie the minimal pool balances/shares to existential deposit.

This should be viable thing after we implement #1007

However this number probably shouldn't change. I would consider setting it to the fallback values so we don't end up in the same situation.

Originally posted by @jak-pan in #1003 (comment)