[NPoS] Remove better solution threshold for unsigned submissions (#2694)

closes https://github.com/paritytech-secops/srlabs_findings/issues/78.

Removes `BetterUnsignedThreshold` from pallet EPM. This will essentially
mean any solution submitted by the validator that is strictly better
than the current queued solution would be accepted.

The reason for having these thresholds is to limit number of solutions
submitted on-chain. However for unsigned submissions, the number of
solutions that could be submitted on average is limited even without
thresholding (calculation shown in the corresponding issue).