Remove commented-out function definitions and delete files with no executable code

[rileyjmurray]

NOTE: this is currently a draft PR. I'll convert it to a normal PR after my other PRs have been merged.

This PR aims to resolve #428. I'll note that some of my open PRs remove some of the same content removed here. I suspect Git will know how the reconcile the differences when those PRs are merged.

At time of opening this PR, the changes are quite likely over-zealous, so I'm happy to roll some changes back. That said, I suggest we be aggressive in removing this kind of unused text. Here's a proposal for how to decide what to actually keep and what to remove:

• We can/should keep simple reference implementations of functions that are important but very complicated.
  • Example: there's an implementation of Levenberg-Marquardt from Wikipedia that I've kept. I think this is valuable because our actual LM code is very complicated, so having a reference is helpful for maintainability.
• We could keep text that's related to memory profiling.
  • Rationale: The question of whether or not to bother trying to profile memory usage in a certain location requires a fair amount of expertise, so I think keeping this kind of text could serve as a flag for suspected hot code paths.
• Commented-out pyx code should be removed unless we have a very good reason for keeping it.

Another source of criteria for figuring out what to keep is to look for a short note at the beginning of some commented-out code block. Often there are notes that say "FUTURE," "Removed," "unused," "REMOVE," "Debugging," etc...

Other, miscellaneous changes

• Removed some unused imports.