Replace scrapers functionality with tasks using prometheus.scrape

as a user
when i create and configure a scraper
then i would like a task created to actually scrape the data

so now that flux has the `prometheus.scrape` endpoint, we can replace the existing scraper functionality with Flux tasks.

Example Flux:
```
import "experimental/prometheus"

option task = {name: "Flux Scrape Data", every: 5s}

prometheus.scrape(url: "http://localhost:9999/metrics")
|> map(fn: (r) =>
({r with scrape_interval: string(v: task.every)}))
|> to(bucket: "flux_scrape_data", org: "influxdata")
```

and updated template dashboard:
[influxdb_2.0_oss_metrics_update.json.zip](https://github.com/influxdata/influxdb/files/3736835/influxdb_2.0_oss_metrics_update.json.zip)

Start by locating the existing scraper functionality and how configured scrapers are executed. Compare that flow with the provided Flux prometheus.scrape task example, then review the linked dashboard template. Done means configured scrapers create tasks that scrape data through prometheus.scrape and the updated dashboard reflects the new task-based flow.

Written by the indexing model from the issue text.