Remove unnecessary translation from merge request model

In `merge_request.rb`, the rebase_lock_message should be translated
but it is not because it is saved to a class constant. This change
generates the message dynamically.

app/models/merge_request.rb

@@ -262,8 +262,6 @@ class MergeRequest < ApplicationRecord
 
   RebaseLockTimeout = Class.new(StandardError)
 
-  REBASE_LOCK_MESSAGE = _("Failed to enqueue the rebase operation, possibly due to a long-lived transaction. Try again later.")
-
   def self.reference_prefix
     '!'
   end
@@ -1514,7 +1512,7 @@ class MergeRequest < ApplicationRecord
     end
   rescue ActiveRecord::LockWaitTimeout => e
     Gitlab::ErrorTracking.track_exception(e)
-    raise RebaseLockTimeout, REBASE_LOCK_MESSAGE
+    raise RebaseLockTimeout, _('Failed to enqueue the rebase operation, possibly due to a long-lived transaction. Try again later.')
   end
 
   def source_project_variables

changelogs/unreleased/118816-remove-translation-from-constant.yml

+---
+title: Fix rebase error message translation in merge requests
+merge_request: 22952
+author: briankabiro
+type: fixed

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -1412,7 +1412,7 @@ describe Projects::MergeRequestsController do
         post_rebase
 
         expect(response.status).to eq(409)
-        expect(json_response['merge_error']).to eq(MergeRequest::REBASE_LOCK_MESSAGE)
+        expect(json_response['merge_error']).to eq('Failed to enqueue the rebase operation, possibly due to a long-lived transaction. Try again later.')
       end
     end
 

spec/requests/api/merge_requests_spec.rb

@@ -2260,7 +2260,7 @@ describe API::MergeRequests do
       put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/rebase", user)
 
       expect(response).to have_gitlab_http_status(409)
-      expect(json_response['message']).to eq(MergeRequest::REBASE_LOCK_MESSAGE)
+      expect(json_response['message']).to eq('Failed to enqueue the rebase operation, possibly due to a long-lived transaction. Try again later.')
     end
   end