Clean up gmx solvate

- Remove unused code.
- Remove help text that did not have corresponding implementation.
- Remove paths in addconf.c that were no longer triggered now that
  gmx insert-molecules doesn't use it (will remove the whole file next,
  though, so careful review may not be warranted).
- Fix most memory leaks outside add_conf (add_conf still leaks several
  MB during the unit tests...).
- Consolidated memory management and reduced the scope of some
  variables.

Change-Id: Ibf570247a7e91bc028401165000142b03baebe4d