Device dehydration v2

- add support for device dehydration v2 i.e. `org.matrix.msc3814`
- run dehydration flows after successfully recovering or creating secrets
- enable said flows based on .well-known `org.matrix.msc3814` config key
- delete previous implementation and helper methods

Riot/Modules/MatrixKit/Models/Account/MXKAccount.h

@@ -360,13 +360,6 @@ typedef BOOL (^MXKAccountOnCertificateChange)(MXKAccount *mxAccount, NSData *cer
 
 #pragma mark - Sync filter
 
-/**
- Call this method at an appropriate time to attempt dehydrating to a new backup device
- */
-- (void)attemptDeviceDehydrationWithKeyData:(NSData *)keyData
-                                    success:(void (^)(void))success
-                                    failure:(void (^)(NSError *error))failure;
-
 /**
  Handle unauthenticated errors from  the server triggering hard/soft logouts as appropriate.
  */