select *
from dba_synonyms
where not exists
 (select 1 from all_objects
  where owner = table_owner
  and   object_name = table_name
 )
order by owner
/
prompt;

select 'drop public synonym ' || synonym_name || ';'
from dba_synonyms
where not exists
 (select 1 from all_objects
  where owner = table_owner
  and   object_name = table_name
 )
and table_owner = 'SYSMAN'
order by owner
/