Remove gmx_ana_index_t::name and unused code.

Remove the name field from gmx_ana_index_t, which was in most cases
NULL, and was a source of multiple ugly hacks in the selection code to
avoid leaking the memory allocated for the name.
Now that SelectionTreeElement has a proper std::string name member,
there is no need for this name field anywhere in the selection code
where the atom groups are handled as selection values.

Also remove unused code from indexutil.* (that would have required extra
effort to not break it).

Change-Id: I14698c17da3b479de4016a6188fcd235514b9e2a