docs: Remove unused docs (#113)

* Revert "docs: add new installation note to migration guide"

This reverts commit a5dbff4.

* Revert "docs: refactor chapter in migration guide"

This reverts commit 106f97d.

* Revert "docs: fix markdown code style in migration guide"

This reverts commit 4d5b151.

* Revert "docs: add obj update examples to migration guide"

This reverts commit 07979d2.

* Revert "docs: add Parse ObjC SDK migration guide (parse-community#392)"

This reverts commit 93c1619.

Author: cbaker6