Documentation: remove link "Subscribing to Git Submodules"
Documentation: remove link "Subscribing to Git Submodules"

The exact same link is found a few lines before
as "Submodules" in "Multi-project management".

Change-Id: Ifacf35cbff76228fd8354fd783d91c64cacbb379
Signed-off-by: Stefan Beller <a7f3b1612c245dae606b3b9617b4f52187fddb65@google.com>

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