Remove the metric aggregator Subtractor interface

[jmacd]

Problem Statement

The Metrics SIG has ruled out specifying that SDKs support a cumulative-to-delta conversion.

Proposed Solution

I advocated for this support not to be standardized because of the complexity it generates, as seen in the OTel-Go SDK. I previously hacked this removal into #1415 to show how much complexity can be removed.

Alternatives

The alternative is to keep this support, but we have determined that all systems we know of would prefer to convert cumulative temporality sums to gauge values than into delta temporality sums.