Remove reserve_recruitement, reserve_volunteers_limit, reserve_recruitement_start_date and reserve_recruitement_end_date

[magul]

Description:

We decided not to introduce reserve recruitement at all in our application, so we need to remove all Offer model attributes that were responsible for it.

Current behaviour:

We have reserve_recruitement, reserve_volunteers_limit, reserve_recruitment_start_date and reserve_recruitment_end_datein ourOffer` model. We store it in database, render it in serializer, show it on frontend and ask for it in forms.

Expected behaviour:

reserve_recruitement, reserve_volunteers_limit, reserve_recruitment_start_date and reserve_recruitment_end_dateis no longer a part ofOffer` model, not used in serializers, not shown and asked for on frontend.

cc: @beerkoo