chore: remove `IBCMiddleware.refundTimeout` as it is no longer used

[ysv]

I was trying to understand meaning of refundTimeout inside IBCMiddleware but didn't find any usage of this attribute except initialization (3 times) and unit test (2 times) - see screenshot.

I'm still wondering what is the reason behind and if it should be present at all.

[Reecepbcups]

cc @jtieri since you did a bit of refund work iirc due to Mandrake

[agouin]

This parameter is no longer used after the introduction of atomic acks. It can be safely ignored (set to arbitrary value) and should be removed in a future release