Remove HTTP admin endpoints

[pawanrawal]

These endpoints are now served via the GraphQL Admin API and hence we are removing the HTTP endpoints. Another PR for updating the docs will follow.

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This change is 

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[MichelDiz]

Just a point, wouldn't it be nice to leave that for compatibility? Some users might not like to be forced to use GraphQL for these operations. Sending curl commands would get bigger and more complex.

e.g:
do this

jq -n '
{ 
  query: "mutation {
  export(input: {
    format: "JSON"
  })
  {
    response {
      Response
    }
  }
}"' | 
curl -X POST -H "Content-Type: application/json" http://localhost:8080/admin -d @- | jq -r

Instead of just

curl 'localhost:8080/admin/export?format=json'

[martinmr]

Reviewed 11 of 11 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain)

[manishrjain]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @pawanrawal)

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graphql/admin/export.go, line 51 at r1 (raw file):

	format := worker.DefaultExportFormat
	fmt.Printf("input: %+v\n", input)

fmt.printfs?