Remove ununsed variables

The warnings are distracting - and the variables are truly obsolete by
now.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix a couple of problems with dive merging

- Dive list no longer has messed up trip entries
- Dive list is now correctly marked as modified after trips are merged
- When data file is closed, the list of selected trips is cleared which
  avoids a potential crash when loading a new data file

Fixes #286

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>