[MRG] Remove lca gather

[ctb]

Fixes #1183 by removing the lca gather command, associated tests, and associated documentation.

As a side note, I think this may be the last code-level change for 4.0...? 🎉

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1307 (54b1bea) into latest (465a06d) will increase coverage by 5.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1307      +/-   ##
==========================================
+ Coverage   88.72%   94.05%   +5.32%     
==========================================
  Files         125       96      -29     
  Lines       18374    14465    -3909     
  Branches     1438     1395      -43     
==========================================
- Hits        16303    13605    -2698     
+ Misses       1824      621    -1203     
+ Partials      247      239       -8     

Flag	Coverage Δ
python	94.05% <100.00%> (+0.08%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/cli/lca/__init__.py	100.00% <ø> (ø)
src/sourmash/lca/__init__.py	100.00% <ø> (ø)
tests/test_lca.py	99.86% <ø> (-0.01%)	⬇️
src/sourmash/lca/__main__.py	53.84% <100.00%> (ø)
src/sourmash/lca/lca_db.py	91.84% <0.00%> (-1.21%)	⬇️
src/core/src/sketch/minhash.rs
src/core/src/ffi/minhash.rs
src/core/src/ffi/mod.rs
src/core/src/from.rs
src/core/src/sketch/nodegraph.rs
... and 22 more

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