Remove more 3b1b files

[eulertour]

Some files and functions missed in #2.

[TonyCrane]

I think some people want to construct their own creature by modifying the code about pi_creature.
Is it better to delete this part of the code?

[leotrs]

@eulertour were you able to run the example scenes after these deletions?

[leotrs]

@tony031218 , the pi creatures are property of 3b1b, so we should totally delete them from this repo. If people are interested in mimicking them, they can always use the original 3b1b repository.

[eulertour]

@leotrs Yes, I was still able to run them.

[Aathish04]

@eulertour were you able to run the example scenes after these deletions?

I wasn't able to run any of the example scenes after the removal of the files in once_useful_constructs. After manimlib.imports was suitably modified, then yeah, everything worked fine.

Why can't we leave once_useful_constructs in, anyway? They're just classes and methods that were useful before and might be in the future.

[eulertour]

@Aathish04 Its last moments are documented here. It isn't part of the master branch anymore so you might have to pull.

[Aathish04]

@Aathish04 Its last moments are documented here. It isn't part of the master branch anymore so you might have to pull.

I'm all for removing stale code. Can we also edit manimlib.imports and remove references to said cleaned up code then?

[eulertour]

I'm not sure what you mean. This PR is already removing for_3b1b_videos from manimlib.imports and once_useful_constructs was removed in 3f5f0cb.

[Aathish04]

I'm not sure what you mean. This PR is already removing for_3b1b_videos from manimlib.imports and once_useful_constructs was removed in 3f5f0cb.

Huh. The imports were still in the manimlib.imports on my computer... odd. It's probably just an issue on my end...