Remove fast-no-conflicts challenges

[danielmitterdorfer]

These challenges are present mostly for historic reasons and don't provide much value by now. Hence, we'll remove them. As we have new index sorting benchmarks (added in #22) that use the fast-no-conflict challenges as a baseline, we'll also need to adapt that they use a different baseline:

The new baselines for the sorted benchmarks will be:

• geonames: append-fast-no-conflicts
• pmc: append-no-conflicts-index-only
• logging: append-no-conflicts-index-only

The other tracks either don't have no index sorting benchmarks or they already use a "non-fast" baseline.