Issue #765 - Delete old overviews of the default metrics

Description

This PR closes #765. This PR closes #757.

As discussed in #764, the old information on the default metrics is outdated. This PR deletes this information. I'll add new info in a new PR (#762).

The PR

Deletes the metrics.rda and metrics-detailed.rds files (and the corresponding metrics data set
Deletes the create-metric-tables.R script
Deletes the Vignettes metric-details.Rmd and scoring-forecasts-directly.Rmd
Deletes a call to the metrics data from the scoringutils.Rmd Vignette
has unintentional(!) unrelated changes: some .Rd docs files got updates when I reinstalled the package (I think that's related to a suggested change during the last PR where we only changed the .R file, not the .Rd file)

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