Changes

chore(pingcap/tiflow): remove engine integration test jobs and pipelines (#3593)

This pull request removes Jenkins pipeline job definitions for
integration tests across multiple branches of the `pingcap/tiflow`
repository. These jobs are no longer needed and have been fully deleted
from the codebase.

Removed pipeline job definitions:

* **General integration test job**:
-
[`jenkins/jobs/ci2/pingcap/tiflow/engine_ghpr_integration_test.groovy`](diffhunk://#diff-61eda0e02d0532f09dc86a05cdb8a6d2e1a355c8f3af4877563c0da78b616c94L1-L83):
Removed the integration test job for general pull requests targeting the
`tiflow` repository.

* **Latest release branch integration test job**:
-
[`jobs/pingcap/tiflow/latest/pull_engine_integration_test.groovy`](diffhunk://#diff-4aa70291b880af4e7c10424b1bf82a4d52eda493fc02d0e2ae81d58162edbe9bL1-L38):
Deleted the integration test job for the latest release branches.

* **Specific release branch integration test jobs**:
- Removed integration test jobs for specific release branches,
including:
    - `release-7.1`
    - `release-7.2`
    - `release-7.3`
    - `release-7.4`
    - `release-7.5`
    - `release-7.6`
    - `release-8.0`
    - `release-8.1`
    - `release-8.2`
    - `release-8.3`
    - `release-8.4`
    - `release-8.5`
    - `release-9.0-beta`
    
---
Close #3589
Close #3594
 (commit: b288a54)