Fix outdated references to soft deletion

We stopped using soft deletion for issues and merge requests in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15789,
and for boards we apparently never used it.

doc/api/boards.md

@@ -466,7 +466,7 @@ Example response:
 ## Delete a board list
-Only for admins and project owners. Soft deletes the board list in question.
+Only for admins and project owners. Deletes the board list in question.
 ```
 DELETE /projects/:id/boards/:board_id/lists/:list_id

doc/api/group_boards.md

@@ -536,7 +536,7 @@ Example response:
 ## Delete a group issue board list
-Only for admins and group owners. Soft deletes the board list in question.
+Only for admins and group owners. Deletes the board list in question.
 ```
 DELETE /groups/:id/boards/:board_id/lists/:list_id

doc/api/issues.md

@@ -790,7 +790,7 @@ the `weight` parameter:
 ## Delete an issue
-Only for admins and project owners. Soft deletes the issue in question.
+Only for admins and project owners. Deletes the issue in question.
 ```
 DELETE /projects/:id/issues/:issue_iid

doc/api/merge_requests.md

@@ -1127,7 +1127,7 @@ the `approvals_before_merge` parameter:
 ## Delete a merge request
-Only for admins and project owners. Soft deletes the merge request in question.
+Only for admins and project owners. Deletes the merge request in question.
 ```
 DELETE /projects/:id/merge_requests/:merge_request_iid