text: Title: [APM] Remove unused transaction breakdown aggregation
  Body: It looks like the following transaction breakdown aggregation is unused:
  
  https://github.com/elastic/kibana/blob/00bb59713ed115343eb70d4e39059476edafbade/x-pack/plugins/apm/server/lib/transactions/breakdown/index.ts#L54-L58
  
  TODO:
   - [ ] Verify whether it's used anywhere. If not remove it.
   - [ ] Rename the function `getTransactionBreakdown` to `getSpanBreakdown`