Prune old Bridge pool merkle trees

[sug0]

we might be able to withgo storing multiple bridge pool merkle trees, as well. we only need to keep trees from the latest root that has been signed with a quorum signature onward. so, suppose the latest merkle tree nonce is $5$, but the latest signed root's nonce is $3$; then, we can delete all the trees signed with a nonce of $0$ through $2$, whilst upholding the liveness of the bridge.

Originally posted by @sug0 in #2043 (comment)