Remove requirement for a codeset to have at least one term

[jamesrwelch]

At the moment, there's a rules that states that a CodeSet must include at least one Term. However, in a new project, we will want to record a Rule (a SNOMED Expression) for a CodeSet to calculate which terms are included (This calculation will happen externally to Mauro). So for these CodeSets we don't want to include any terms explicitly.

[olliefreeman]

@jamesrwelch We actually removed this restriction on the backend back in MDC world. The remaining restriction is in the UI only, therefore transferring to the UI.