Clarify status of backup implementation

[Johennes]

In a chat with @ara4n yesterday I learnt that the legacy backup has actually been implemented in matrix-sdk-crypto and isn't planned to be added here anymore. I suppose MSC4048 will be implemented there, too, then which is why I have removed its reference as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.56%. Comparing base (4ef989c) to head (7b3e5f7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #155 +/- ## ======================================= Coverage 90.56% 90.56% ======================================= Files 34 34 Lines 1908 1908 ======================================= Hits 1728 1728 Misses 180 180 ```

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[dkasak]

We avoided implementing primitives for the existing backup in vodozemac because the design is known to be flawed. Therefore, in my opinion it's not a given that none of the improved design (MSC4048) will end up in vodozemac. The deciding factor for that should be whether something is a "primitive" or foundational cryptographic construct, in which case it belongs in vodozemac, or if it is more of a Matrix layer feature, in which case it should be part of matrix-sdk-crypto.

[Johennes]

Ah, I see. Thanks for clarifying.