Dive sites: don't delete unused dive sites on save

Unused dive sites were deleted on save. This clashed with the undo
system in the following scenario:

1) Delete single-use dive site.
2) Save (dive site deleted)
3) Undo (reference to freed dive site)

Therefore, as a quick-fix, keep the referenced dive site around.
Note that this also means that empty dive sites must not be
deleted, as it might refer to a dive in the undo system. Instead
only clear references to empty dive sites in the global dive
table. Factor this functionality out, as it was common to the
XML and git savers.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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