ci(tikv/migration): remove pull_unit_test job from ci repo

[wuhuizuo]

Background

pull_unit_test for tikv/migration should be removed from this CI repository as a follow-up item split from #4272.

Scope

• Remove the pull_unit_test presubmit entry from prow-jobs/tikv/migration/latest-presubmits.yaml
• Remove Jenkins job DSL jobs/tikv/migration/latest/pull_unit_test.groovy
• Remove pipeline pipelines/tikv/migration/latest/pull_unit_test.groovy
• Remove pod template pipelines/tikv/migration/latest/pod-pull_unit_test.yaml

Notes

• Keep integration jobs unchanged
• This is a sub issue split from feat(tikv/migration): migrate CI artifacts from fileserver to OCI registry #4272
• Related parent issue: fileserver: migrate artifacts for tikv/migration #4215

[wuhuizuo]

Depends on tikv/migration#431