Remove shallow-water IC fields from periodic hex meshes

[mgduda]

This merge removes shallow-water initial conditions from periodic hex meshes.

The periodic_hex mesh generator previously added fields to the grid.nc
file to serve as placeholders for ICs for the shallow-water model.
However, these fields are not part of the MPAS mesh specification, and
have been removed. Specifically, the following fields are deleted in
this commit:

  fEdge
  fVertex
  h_s
  u
  uBC
  v
  h
  vh
  circulation
  vorticity
  ke
  tracers

Additionally, the dimensions nVertLevels and nTracers have also been
removed.

[mgduda]

I'm actually not sure which MPAS cores were using the uBC field that has been removed in this PR, but in case its absence would be a problem, we could perhaps leave it in the mesh file.

[matthewhoffman]

@mgduda , this looks great to me. The presence of those fields was always a minor annoyance to me. I believe that all of our cores create new mesh files from the output of periodic_hex and don't actually use the meshes it generates directly.