chore: remove field ephemeral from runner registration response (#11350)

Remove the field `ephemeral` from the response to runner registration requests made using the HTTP API (POST to `/repos/{owner}/{repo}/actions/runners` and friends) that was introduced with https://codeberg.org/forgejo/forgejo/pulls/9962. The client already knows that it requested an ephemeral runner. Therefore, the information is redundant.

It can be included again should a compelling use case arise.

This part of the HTTP API hasn't been released yet. Therefore, it is safe to remove the field.

## Checklist

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### Tests for Go changes

(can be removed for JavaScript changes)

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Tests for JavaScript changes

(can be removed for Go changes)

- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11350
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>
Co-committed-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>

modules/structs/runner.go

@@ -25,8 +25,7 @@ type RegisterRunnerOptions struct {
 // RegisterRunnerResponse contains the details of the just registered runner.
 // swagger:model
 type RegisterRunnerResponse struct {
-	ID        int64  `json:"id" binding:"Required"`
-	UUID      string `json:"uuid" binding:"Required"`
-	Token     string `json:"token" binding:"Required"`
-	Ephemeral bool   `json:"ephemeral" binding:"Required"`
+	ID    int64  `json:"id" binding:"Required"`
+	UUID  string `json:"uuid" binding:"Required"`
+	Token string `json:"token" binding:"Required"`
 }