config: remove hashes from credentials.password

In the initial credentials schema, the hashes were supposed to give a
way to have passwords out of plain-text config file.
Later, it was decided to remove this feature, because this way of
authorisation is inferior to the one with auth service and tokens,
but the latter is out of scope for current config development.
This patch removes `credentials.password.{sha1,sha256}` and moves
plain password from `credentials.password.plain` to `credentials.password`.

Part of #8967

NO_DOC=tarantool/doc#3544 links the most actual schema,
       no need to update the issue.
NO_CHANGELOG=removed feature was not released yet