Remove unused requirements-sparseattention

[segyges]

• Removes this requirements file
• Removes reference to it in the dockerfile
• Also removes protobuf from the dockerfile since it is apparently unused and I noticed it
• Causes confusion and possible problems while installing since it will force a downgrade of Triton
• If downgrading Triton is actually essential I am wrong and this PR should be rejected.

[segyges]

This needs to cook more, apparently protobuf is actually used, I think we probably want to change the version of flashattention.

[Quentin-Anthony]

Removing sparse attn from Dockerfile + removing the Dockerfile Flash mismatch was just handled in #1175, and as you mentioned we still need protobuf.

I would, however, still be interested in just updating the version of Flash to be consistent with the Dockerfile + removing requirements/requirements-sparseattention.txt