[bug] remove mac from secrets file

This is how a secret was stored in the secrets json file:

  * each secret is symmetrically encrypted amd MACed with keys derived from
    the user's passphrase.

  * the encrypted secrets dictionary is then MACed with another key derived
  * from the user's passphrase.

  * each key is derived using scrypt and a unique random salt.

There are disadvantages to this approach:

  * repeating scrypt many times is a waste of time.

  * an attacker could crack whichever has weaker parameters, if they get out
    of sync.

  * if an attacker can modify the secret in a way it is good to decrypt the
    database, then she can also modify the MAC.

The solution for this is:

  * completelly eliminate the MAC from the storage secrets file.

  * attempt to decrypt the database with whatever is got from the decryption
    of the secret. If that is wrong, report an error.

Closes #6980.

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