Remove unused Units, Scope, and MetricGroup conventions

[lmolkova]

Removes unused UnitSemanticConvention, MetricGroupSemanticConvention and ScopeSemanticConvention

(part of #303)

[joaopgrassi]

Hum, is MetricGroupSemanticConvention really not used? I thought it is part of the whole metrics semconv gen. @jamesmoessis I think introduced this initially.

[lmolkova]

Hum, is MetricGroupSemanticConvention really not used? I thought it is part of the whole metrics semconv gen. @jamesmoessis I think introduced this initially.

nope, it's not and I don't quite understand what it is, but happy to learn :)

[jamesmoessis]

It might've been used initially to group metric attributes that weren't attached to specific metrics. However, if the direction has gone elsewhere and not used it, fine to delete :)

[lmolkova]

We're moving over to weaver to generate semantic conventions markdown and code and don't intend to release new version of build-tools.

So I'm going to close this PR for the time being.