Dive site: un-globalize displayed_dive_site

The global object displayed_dive_site is used a a backing-store
by the dive-site-edit widget. All external accesses were removed,
therefore make the object local to the widget.

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

core/dive.c

@@ -18,7 +18,6 @@
  * it's used in the UI, but it seems to make the most sense to have it
  * here */
 struct dive displayed_dive;
-struct dive_site displayed_dive_site;
 
 struct tag_entry *g_tag_list = NULL;