Remove dependency on leveldb (#4213)

core/vm/lightclient/iavl/.gitignore

core/vm/lightclient/iavl/CHANGELOG.md

core/vm/lightclient/iavl/CONTRIBUTING.md

core/vm/lightclient/iavl/LICENSE

core/vm/lightclient/iavl/Makefile

core/vm/lightclient/iavl/PERFORMANCE.md

core/vm/lightclient/iavl/POEM

core/vm/lightclient/iavl/README.md

core/vm/lightclient/iavl/doc.go

core/vm/lightclient/iavl/key_format.go

core/vm/lightclient/iavl/key_format_test.go

core/vm/lightclient/iavl/proof.go

core/vm/lightclient/iavl/proof_iavl_absence.go

core/vm/lightclient/iavl/proof_iavl_value.go

core/vm/lightclient/iavl/proof_path.go

core/vm/lightclient/iavl/proof_range.go

core/vm/lightclient/iavl/version.go

core/vm/lightclient/iavl/wire.go

core/vm/lightclient/multistoreproof.go

core/vm/lightclient/utils.go