Drop Webhooks from project import/export config

Webhook URLs were recently encrypted in the database via
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21645, and as a
result they are no longer exported. When an exported project with
Webhooks is imported, the project import service will fail with
`URI::InvalidURIError: bad URI(is not URI?)` due to a blank URL.
We avoid this by disabling the export/import of Webhooks in
the first place.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53513

changelogs/unreleased/sh-drop-webhooks-project-export.yml

+---
+title: Drop Webhooks from project import/export config
+merge_request: 24121
+author:
+type: fixed

doc/user/project/settings/import_export.md

@@ -71,6 +71,7 @@ The following items will NOT be exported:
 - Build traces and artifacts
 - Container registry images
 - CI variables
+- Webhooks
 - Any encrypted tokens
 
 ## Exporting a project and its data

lib/gitlab/import_export/import_export.yml

@@ -63,7 +63,6 @@ project_tree:
   - :triggers
   - :pipeline_schedules
   - :services
-  - :hooks
   - protected_branches:
     - :merge_access_levels
     - :push_access_levels
@@ -156,12 +155,6 @@ excluded_attributes:
     - :reference
     - :reference_html
     - :epic_id
-  hooks:
-    - :token
-    - :encrypted_token
-    - :encrypted_token_iv
-    - :encrypted_url
-    - :encrypted_url_iv
   runners:
     - :token
     - :token_encrypted