remove low-workload benchmarks that are flaky

[varun-sundar-rabindranath]

Summary:
This PR removes the following benchmarking runs,

• Sparse 2:4 : prompts 300, qps 1
• Sparse 50 % : prompts 150, qps 0.5

Nightlies that reported flakiness / regression:

• https://github.com/neuralmagic/nm-vllm/actions/runs/8444603631/job/23138489705
• https://github.com/neuralmagic/nm-vllm/actions/runs/8475302354/job/23229958270
• https://github.com/neuralmagic/nm-vllm/actions/runs/8486867771/job/23258658412

On the nightlies we trigger a perf. regression alert when the value change is more than 10%. For the cases being removed, the variance is in the range of [10% - 20%]

Test:
None

[varun-sundar-rabindranath]

median_ttft_ms (the flaky metric) graph of the cases being removed,