md: Kill footnote on Repairable Threshold Schemes

What we have built is, in fact, a repairable threshold scheme, where
other participants can help repair a share of a broken participant.
(Our scheme is even better because a single peer suffices.) The footnote
was a bit misleading in that it says that this could be possible without
the seed. But without some secret, the participant can't authenticate to
peers anymore, no matter what repairable threshold scheme will be used.

(If we changed the scheme to store also the "self-share" in encrypted
form in the recovery data, then we'd obtain a scheme where the seed is
really only used for "authentication", namely decrypting the secshare.)

README.md

@@ -487,13 +487,6 @@ and we believe that a general recommendation is not useful.
 
 TODO Explain second purpose of recovery data, say that participants are responsible for convincing others
 
-[^advanced-recovery]: (TODO Do we want to kill this? This is so far down the road with unclear security assumption (semihonest) that I'm not convinced that we want to talk about this at all.)
-In theory, there are advanced strategies to recover the secret share of a participant with the help of other participants, even if the seed is lost.
-For example, if threshold-many participants are cooperative, it may be to possible to use the "Enrolment Repairable Threshold Scheme" described in [these slides](https://github.com/chelseakomlo/talks/blob/master/2019-combinatorial-schemes/A_Survey_and_Refinement_of_Repairable_Threshold_Schemes.pdf).
-(TODO proper citation)
-This scheme requires no additional backup or storage space for the participants.
-These strategies are out of scope for this document.
-
 ### Threat Model and Security Goals
 
 Some participants, the coordinator, and all network links may be malicious, i.e., controlled by an attacker.