[BUG]: stray dive-sites after parsing ssrf/xml

[Lubomir I. Ivanov]

Dennis Hansen has reported a bug at the google groups forum and then
reported the same issue at Github:
https://github.com/Subsurface-divelog/subsurface/issues/786

the SSRF file he provided has some dive sites without names and no
dives are using them:

------------------
<site uuid=' c39aa1c' gps='26.301216 34.865268'>
</site>
...snip...
<site uuid=' 400dee6' gps='25.321884 34.742533'>
</site>
...
------------------

in MapWidgetHelper::reloadMapLocations() we do not skip such and
markers are added for them.
judging from the Marble port that i did, we had the same behavior there.

also, not exactly sure how this ended up happening, but i need some
decision making here:
- should we discard these at the parsing stage?
- should we discard this at the saving stage?
- should the map widget *ignore* such dive sites that are not linked
to dives and not create markers for them?
- should we leave this as is? but perhaps tell the MainTab to deselect
the last dive, because at them moment clicking such a marker doesn't
do that.

thanks
lubomir
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