Remove dead code for assigned voiceartist list in the opportunity model and language_codes_needing_voice_artists.

[DubeySandeep]

Currently while creating an opportunity model we aren't populating the assigned_voice_artist_in_languages as there aren't any process of assigning voice artist. Once the voiceover application functionality is implemented change the opportunity model creation method such that it also populates the assigned_voice_artist_in_languages with the required data.

This will also remove the TODO tag used in the codebase here:

oppia/core/domain/opportunity_services.py

Lines 144 to 147 in 25e62cd

	# TODO(DubeySandeep): Once the voiceover application functionality is
	# implemented change this method such that it also populates the
	# assigned_voice_artist_in_languages with the required data.

[seanlip]

Adding a note from #13912 too: "Currently language_codes_needing_voice_artists in core/domain/opportunity_services.py is based off the translation status of written translations. This doesn't quite make sense for voiceovers."

As an update -- we should just get rid of these fields / dead code, since they are unneeded, as we are not going to be handling voiceovers through the contributor dashboard.