Remove github-app-cloud feature flag
Created by: miveronese
Ticket: CLOUD-292
This PR removes the code parts/checks that use the github-app-cloud
feature flag, in both backend and frontend.
After the PR is merged, we can remove the feature flag from the database.
Test plan
The updated backend unit test can be run with:
go test ./enterprise/cmd/frontend/internal/app/... -v
All other unit tests should pass along with the integration tests.