Support remove liquidity from Element pools

[brunoguerios]

from @agualis

I see in sentry that some users tried to remove from this pool but they are getting an error (pool type ELEMENT is not supported). I’m not familiar with this pool type. Anyone knows how to enable removes?

[brunoguerios]

Likely possible by simply reusing StablePool implementation (assuming they use the same encoder) - double check with integration test

[johngrantuk]

Unless this is SUPER simple I don't think we should spend time on it - the protocol itself don't support it on their UI and have provided alternative instructions on how to remove.