docs/process: branching-checklist: Remove section about the cambridge colo

docs/process: branching-checklist: Remove section about the cambridge colo

The cambridge colo was internal to Citrix (now Cloud) and hasn't been used
by the community for a while.

So remove the section.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: George Dunlap <george.dunlap@cloud.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
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    Changes in v2:
        - Add George's acked-by
        - Add Henry's reviewed-by