The only external caller of add_single_dive() used it to append a
dive to the global dive list. Rename the function accordingly and
remove the index parameter.
The internal caller can use the local insert_dive() function, which
doesn't consider selection. That shouldn't be a problem, as the
caller is doing import.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
