Changes

refactor(jobs/pingcap/tidb): remove merged build and test jobs for tidb trunk branch (#3562)

The following postsubmit jobs can be removed because they are covered by
a required presubmit job:

| Postsubmit Job Name | Remove? | Reason |

|--------------------------------------|:-------:|----------------------------------------------|
| merged_integration_e2e_test | ✅ | Covered by required presubmit |
| merged_integration_ddl_test | ✅ | Covered by required presubmit |
| merged_build | ✅ | Covered by required presubmit |
| merged_mysql_client_test | ✅ | Covered by required presubmit
 (commit: 72cb0d3)