Move Tactic Validation for Techniques from ATT&CK Workbench Frontend to the Backend

[adpare]

ATT&CK Worbench frontend should move validation and existence of a tactic for a technique to the backend and let the ATT&CK Data Model handle validation. Below validation should be remove from src/app/classes/stix/technique.ts

          // only check tactics if object is not a draft
          result.errors.push({
            field: 'tactics',
            result: 'error',
            message: 'object has no tactics',
          });
        }

Related ATT&CK Data Model issue