[WIP] Reduce wasm surface by removing index

[luizirber]

For greyhound.sourmash.bio and similar browser-side demos I've been using a subset of sourmash that is compilable to webassembly. To make sure that new features and dependencies are added that break what webassembly supports I added a CI check that tries to build a JS package using wasm-pack.

But... that limits quite a bit what we can add to sourmash. We can't have C/C++ deps, for example. It also holds back some parallel stuff because rayon doesn't work well on wasm without many tweaks.

So, this PR removes some submodules from wasm (using configuration flags) so we can explore more before commiting to maintaining it in the long run, namely:

• index-related code, because there are other limitations for wasm32 (like not being able to address more than 2GB of RAM) that make it not as useful to have indices in the browser at the moment
• Nodegraph, with the goal of unlocking [WIP] Using bitmagic for internal Nodegraph representation #1221 eventually

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?

[luizirber]

#1643 is a better solution.