Remove cosmos/iavl from lazyledger-core to stop dependency graph cycle about celestia-core

[tac0turtle]

Within lazyledger-core we can remove Iavl. It is used for testing light client queries but we will not be supporting these form of proofs (I believe). I can open a PR later today.

[evan-forbes]

I also found another import cycle that stops go modules from updating the modules of lazyledger-core with go get -u ./..., that's caused by the recent switch from github.com/liamsi/merkletree to github.com/lazyledger/merkletree. Luckily, the import cycle is simpler, and can be fixed by updating the nmt module imported in lazyledger-core using go get -u github.com/lazyledger/nmt.

[liamsi]

It makes sense to remove IAVL from ll-core. AFAR, it is only used in tests only.

Linking related issues & discussions here for context:

• #112 (comment)
• tendermint/tendermint#5694
• cosmos/iavl#46
• cosmos/iavl#125 (comment)