Remove some constraints from DB

[etj]

• index on dcatapit_vocabulary.text:
  useless, since no search on db is made on that field; harmful, since some text may be too long to index.
• unique on dcatapit_subtheme.path
  doesn't seem to be used except for sorting (when the primary sorting is for parent id)
  some subthemes are using the same label, so the mapping can't be loaded

[etj]

Also see https://github.com/geosolutions-it/ckanext-multilang/issues/39

[etj]

ALTER TABLE public.dcatapit_subtheme DROP CONSTRAINT IF EXISTS dcatapit_subtheme_path_key;
DROP index IF EXISTS public.ix_dcatapit_vocabulary_text;