Fix some interactions with Encore

Encore's `onBeforeTurn` event handler was causing problems with
interactions with Taunt and Torment (and possibly other things)
by changing move decisions from Struggle to the Encored move.

The `onBeforeTurn` event handler is not a necessary part of the
Encore implementation because `battle.parseChoice` already calls
`pokemon.getValidMoves`, which in turn calls `pokemon.getMoves`,
which checks if the selected move is disabled. The `encore` effect
already disables all moves other than the Encored one, so they
cannot be selected even without the `onBeforeTurn` handler.

In the result, this commit deletes the `onBeforeTurn` handler.