Cleanup: [Fluidsynth] Remove fluid_player_join

The function fluid_player_join in the library is broken beyond compare for the
usecases it was used for (see their #872). It does not wait until it is safe
to delete the player, so it is up to the end user to ensure that.

For OpenTTD we acquire a lock before fluid_synth_write_s16 and we acquire the
same lock in the stop function. So, only one of the functions can be doing its
thing, meaning we do not need to wait for the player to be stopped as it
cannot be doing anything as we prevent that by the lock.