Fix duplicated branch creation/deletion Web hooks/service notifications when using Web UI
Similar to 423d2d62, except duplicates occurred only if a Web service (e.g. Slack) were
configured.
When deleting a branch, this is what was happening:
1. DeleteBranchService calls execute_hooks and execute_services
2. The call to repository.rm_branch triggers the GitHooksService.
3. This, in turn, calls GitPushService and then calls the same hooks/services again.
5145706c now makes it no longer necessary for DeleteBranchService and CreateBranchService to execute
the branch hooks/services. Note that tags behave differently in GitTagPushService and GitPushService
is not called.
Closes #10330
See merge request !2763
