Remove code for initiating attestations via the propagation pallet / offchain worker

[ameba23]

We have some code which is not currently used for anything since we changed the way attestations are initiated.

The chainspec, propagation pallet, and threshold signature server have code relating to triggering attestations which is no longer needed, including two ignored tests in entropy-tss.

Im not sure if we should go ahead and remove this code, or wait until we are totally clear on our attestation design. I feel like there is still a little bit of uncertainty as to whether more frequent attestation will be needed.

If this still isn't clear at the point of making the next release i would remove it.

Related to #982