[SDK] View should not have a unit

[marcalff]

Class View has a unit member.

This member seems unused.

Note that the SDK spec only mentions units in the instrument selection criteria, not in stream configuration.

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#instrument-selection-criteria

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#stream-configuration

[marcalff]

Related to #2236.

A unit was added:

• in the meter selector
• in the view

but it seem, looking at the spec, that only the meter selector needs unit.