[MRG] remove mentions of ijson and khmer

[ctb]

This updates the docs to remove mentions of these two no-longer-needed packages.

• 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 #1140 into master will increase coverage by 25.16%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1140       +/-   ##
===========================================
+ Coverage   67.86%   93.02%   +25.16%     
===========================================
  Files          24       74       +50     
  Lines        3314     5810     +2496     
===========================================
+ Hits         2249     5405     +3156     
+ Misses       1065      405      -660     

Flag	Coverage Δ
#rusttests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/lib.rs
src/core/src/errors.rs
src/core/src/sketch/nodegraph.rs
src/core/src/index/search.rs
src/core/src/ffi/utils.rs
src/core/src/index/mod.rs
src/core/tests/minhash.rs
src/core/src/index/sbt/mod.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/signature.rs
... and 88 more

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