remove unused function delete_failed_submission

[zubairraeen]

Merge Checklist

Please check all of the following items before merging

• [ ] No new WPCS issues
• [ ] No notices, warnings or errors in debug.log
• [ ] No sonar cloud issues
• [ ] No errors while running automated tests

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud