wtxmgr/db: remove LatestVersion const in favor of getLatestVersion
wtxmgr/db: remove LatestVersion const in favor of getLatestVersion

In this commit, we can remove the LatestVersion constant as it's no
longer needed. Instead, we'll now define the latest version as the last
entry in the slice of versions previously defined.

btcsuite/btcwallet,btcsuite/btcwallet