Remove BIGSI and SBT code from Rust layer

[luizirber]

(spun off #2230, might help with #2665)

BIGSI and SBT are prototype-level code (and some of the first Rust code I wrote...), and mostly makes it harder to change/refactor other parts of the codebase.

Can bring it back later in the future if needed, but mastiff cover many of the same use cases.

(CTB note Sep 17 2023: SBTs are still available in the Python code and in sourmash!)

[codecov]

Codecov Report

Merging #2732 (155d39e) into latest (d0e3726) will increase coverage by 0.55%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           latest    #2732      +/-   ##
==========================================
+ Coverage   85.34%   85.89%   +0.55%     
==========================================
  Files         133      130       -3     
  Lines       15193    14788     -405     
  Branches     2614     2614              
==========================================
- Hits        12966    12702     -264     
+ Misses       1926     1785     -141     
  Partials      301      301              

Flag	Coverage Δ
hypothesis-py	25.80% <ø> (ø)
python	92.83% <ø> (ø)
rust	49.81% <0.00%> (-2.24%)	⬇️

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Files Changed	Coverage Δ
src/core/src/index/mod.rs	0.00% <0.00%> (-39.19%)	⬇️

... and 6 files with indirect coverage changes

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