Animation's `remover` parameter does not work

## Description of bug / unexpected behavior

Setting `remover=true` on the Animation class should remove the mobject. It doesn't.

## Expected behavior

## How to reproduce the issue

Code for reproducing the problem

```py
Paste your code here.
```

## Additional media files

Images/GIFs

## Logs
Terminal output

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```

## System specifications

System Details

- OS (with version, e.g Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
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```

LaTeX details

+ LaTeX distribution (e.g. TeX Live 2020):
+ Installed LaTeX packages:

FFMPEG

Output of `ffmpeg -version`:

```
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```

## Additional comments

See https://github.com/ManimCommunity/manim/pull/1374#discussion_r620557421

Start by reading the Animation class and the discussion linked from PR #1374 to understand how the remover parameter is intended to behave. Reproduce the reported case with remover=true and verify that the animated mobject is removed when the animation finishes.

Written by the indexing model from the issue text.