bracket-push: outdated "from trait" tag

[Emerentius]

In the recent removal of the Brackets struct requirement in the bracket push exercise, this tag was left in, which caused confusion and more "object-oriented" solutions to be submitted.

[petertseng]

For the person who wants to fix this, this person should remove https://github.com/exercism/rust/blob/master/config.json#L608 line. This person should reference #620 as the original removal