Fix #387: Clean up backend type annotations page.

[seanlip]

This fixes https://github.com/oppia/oppia-web-developer-docs//issues/387 by removing info on the wiki page related to the no-longer-existent mypy denylist.

[Ash-2k3]

Thanks @seanlip. Just one thought - Should we also mention the limitation of the mypy and when it might not work as expected ?

Something like - "mypy is a static type checker for Python, designed to enforce type safety by verifying that types declared in type annotations are respected throughout the code. However, mypy has limitations when dynamic typing is involved, especially with the use of functions like setattr that dynamically set attributes at runtime."

[seanlip]

@Ash-2k3 Hm, I think not. Outside tests, we only use setattr in two places and I don't think we should normalize it. If anything I think we should get rid of the usage of setattr.

[Ash-2k3]

Okay, in that case, let's file a bug to drop the usage of setattr, as discussed in our Google Chat conversation.

[seanlip]

@Ash-2k3 Done -- oppia/oppia#21293.

[Ash-2k3]

Thanks, LGTM!