omega-h: fix dependencies (#9552)

* omega-h: fix dependencies

1. Remove Gmodel altogether. Omega_h doesn't
   actually call Gmodel for anything, it was
   more or less just doing additional unit
   testing for Gmodel

2. Make Trilinos off by default. Most codes do
   not need the ParameterList interface, and
   Kokkos through Trilinos is not well explored
   in Spack

3. Use modern CMake style to choose the zlib
   dependency