consider eliminating all of the identifier version removal in lca and tax

[ctb]

when I first implemented the LCA stuff, I was young and foolish and thought there was a point to allowing flexibility in removing identifier versions - e.g. converting GCF_000422665.1 into GCF_000422665 for purposes of lineage mapping.

But, in practice, this is really not used and just massively complicated the code base.

I think we should remove it completely and just standardize on identifiers for LCA and taxonomy being the first space-separated value in the signature name. We could even make it a SourmashSignature property, .ident. 🤔

A responsible way to do this would:

• for sourmash v5, make --keep-identifier-versions=True the default everywhere; this would be a breaking change;
• for sourmash v6, make remove support for keep-identifier-versions altogether.

This musing is occasioned by #1808 where the implementation of LCA_SqliteDatabase is made more confusing by this identifier stuff.

[ctb]

plan: fix in tax, switch to using tax taxonomy loading in lca index per #2198, done.