Changes

opt(pipelines/pingcap/tidb): remove outdated merged and pull tiflash test pipelines and their configurations (#3569)

Deletes the following files related to the merged and pull tiflash test
pipelines, which are no longer in use:
- merged_tiflash_test.groovy
- pull_tiflash_test.groovy
- pod-merged_tiflash_test.yaml
- pod-pull_tiflash_test.yaml

These removals are part of a cleanup effort to streamline the CI process
and eliminate obsolete configurations.

Close #3568

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Signed-off-by: wuhuizuo <wuhuizuo@126.com>
 (commit: b40684a)