Remove spec for in-memory exporters

Currently, the spec considers the in-memory exporter a push metric exporter:

In-memory Metrics Exporter is a Push Metric Exporter which accumulates metrics data in the local memory and allows to inspect it (useful for e.g. unit tests).

The original intent of #2415 was to define a default interval to push metrics from a periodic metric reader paired with the in-memory exporter. The interval I proposed was infinite - that is, by default, an explicit force flush would be required to collect metrics. This raised the question of whether the in-memory exporter is actually a pull metric exporter.

An argument was offered in favor of leaving it as a push exporter https://github.com/open-telemetry/opentelemetry-specification/pull/2415#issuecomment-1068327497.

A number of folks felt that providing it as pull metric exporter offers better ergonomics for the primary use case which is unit tests. One suggestion (https://github.com/open-telemetry/opentelemetry-specification/pull/2415#issuecomment-1069335024) is that both push and pull may be valuable.