[CI] Simplify Jenkins pytest execution

[tqchen]

This PR simplifies Jenkins pytest execution around standard pytest-xdist behavior.

• Runs each already-filtered CPU/GPU suite once with -n auto; the broad suite keeps load-group scheduling because its order-sensitive cases require it.
• Removes external sharding, wrapper/profile code, JUnit XML generation and publication, the skipped-test XML consumer, obsolete suite naming, and orphaned helpers.
• Retains one inert task_clear_pytest.sh entry point only because PR jobs evaluate their Jenkinsfile from the trusted base branch before checking out the PR; it performs no cleanup or reporting and can be removed after this pipeline lands.
• Corrects stale broad-suite paths and explicit target guards, and migrates a scalar stride test to the current T.handle pointer semantics while preserving its negative lowering check.
• Prevents nested MetaSchedule/XGBoost unit tests from multiplying CPU fanout without serializing the full suite.
• Builds only the tvm_runtime target for the secondary GPU configuration and removes its unconsumed gpu2 artifact upload.

The result reduces parallelism to one layer managed by pytest-xdist while preserving GPU filtering and native failure visibility.

[gemini-code-assist]

Code Review

This pull request simplifies the CI pipeline by removing test sharding, the custom run_pytest wrapper, and the skipped tests reporting bot. Instead, it transitions to running tests directly via pytest -n auto using pytest-xdist for parallel execution. To prevent CPU throttling under parallel execution, several meta-schedule tests have been updated to limit thread/core usage. Feedback on the changes suggests making the test target validation in task_python_unittest.sh more robust by checking for general existence (-e) rather than strictly requiring a directory (-d), which would allow individual test files to be specified in the future.

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