Remove duplicate .nr members from selection structs.

Remove duplicate .nr members from selection structs.

gmx_ana_indexmap_t::nr and gmx_ana_pos_t::nr were leftovers from C times
when these structures were directly exposed to user tools, and the
number of positions was provided as convenience in these members.
Both duplicated the value of (m.)mapb.nr.

Change-Id: If5211d7b513eb3ebcd87d4293d19c19c78952564