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