core: move freestanding functions into divelog class

There were only two of them, from the time C-code had to access
the divelog: clear_divelog() and delete_single_dive().

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

subsurface-downloader-main.cpp

@@ -109,7 +109,7 @@ int main(int argc, char **argv)
 		printf("No log files given, not saving dive data.\n");
 		printf("Give a log file name as argument, or configure a cloud URL.\n");
 	}
-	clear_divelog(&divelog);
+	divelog.clear();
 	parse_xml_exit();
 
 	// Sync struct preferences to disk