Removes G_EARTH from OBCs

- The parameter G_EARTH is unnecessarily read and logged by OBCs.
  This is normally innocuous except PR mom-ocean#1088 rearranges the call
  order between OBCs and vertGrid which puts G_EARTH in the wrong
  block in the doc files.
- This removes G_EARTH from OBCs so it is not logged and the two
  outstanding PRs (mom-ocean#1088 and mom-ocean#1089) don't move G_EARTH to the
  wrong place.
- This PR on its own does not change the doc files prior to mom-ocean#1088.

Author: adcroft

src/core/MOM_open_boundary.F90

@@ -240,7 +240,6 @@ module MOM_open_boundary
   logical :: zero_biharmonic = .false.                !< If True, zeros the Laplacian of flow on open boundaries for
                                                       !! use in the biharmonic viscosity term.
   logical :: brushcutter_mode = .false.               !< If True, read data on supergrid.
-  real :: g_Earth                                     !< The gravitational acceleration [m s-2].
   logical, pointer, dimension(:) :: &
                    tracer_x_reservoirs_used => NULL() !< Dimensioned by the number of tracers, set globally,
                                                       !! true for those with x reservoirs (needed for restarts).
@@ -347,9 +346,6 @@ subroutine open_boundary_config(G, US, param_file, OBC)
   call get_param(param_file, mdl, "OBC_NUMBER_OF_SEGMENTS", OBC%number_of_segments, &
                  "The number of open boundary segments.", &
                  default=0)
-  call get_param(param_file, mdl, "G_EARTH", OBC%g_Earth, &
-                 "The gravitational acceleration of the Earth.", &
-                 units="m s-2", default = 9.80)
   call get_param(param_file, mdl, "OBC_USER_CONFIG", config1, &
                  "A string that sets how the open boundary conditions are "//&
                  " configured: \n", default="none", do_not_log=.true.)