[api-logs] remove `ProxyLogger` export

[pichlermarc]

Description

We don't need to export ProxyLogger according to the Logs API specification, so it's best we remove them before integrating the package in to @opentelemetry/api to minimize API surface.

This issue is considered done when

• export has been removed here:

  opentelemetry-js/experimental/packages/api-logs/src/index.ts

  Line 25 in 7a9b049

  export { ProxyLogger } from './ProxyLogger';

• @opentelemetry/api-logs internal usages have been updated to reference the file directly