Removed write_plots and write_netcdf from some NCL diagnostics

[schlunma]

This PR removes the write_plots and write_netcdf options (that will be removed in v2.4) from many NCL that would crash without them. I tested all affected recipes successfully.

It also adds an exception to the automatic addition of plot_file to the provenance entry when plot_file == "n/a" (which is used by some NCL diagnostics).

• Related to Options write_plots and write_netcdf will be removed in ESMValCore v2.4 #2005

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[bouweandela]

Thanks!