Deallocate G%Domain in destroy_dyn_horgrid

  Added calls to deallocate G%Domain and G%Domain%mpp_domain in
destroy_dyn_horgrid.  This eliminates a potential memory leak. It had not been
done before because there was some concern that these pointers might have been
set to point to an element of another type, but this is now all being done via a
copy, so they can (and should) be deallocated.  All answers are bitwise
identical.

src/framework/MOM_dyn_horgrid.F90

@@ -125,7 +125,6 @@ module MOM_dyn_horgrid
     CoriolisBu    ! The Coriolis parameter at corner points, in s-1.
   real, allocatable, dimension(:,:) :: &
     dF_dx, dF_dy  ! Derivatives of f (Coriolis parameter) at h-points, in s-1 m-1.
-!  real :: g_Earth !   The gravitational acceleration in m s-2.
 
   ! These variables are global sums that are useful for 1-d diagnostics
   real :: areaT_global  ! Global sum of h-cell area in m2
@@ -350,11 +349,8 @@ subroutine destroy_dyn_horgrid(G)
   deallocate(G%gridLonT) ; deallocate(G%gridLatT)
   deallocate(G%gridLonB) ; deallocate(G%gridLatB)
 
-  ! Do not deallocate G%Domain%mpp_domain or deallocate(G%Domain) because
-  ! these are pointers to types used elsewhere.
-  G%Domain => NULL()
-  ! deallocate(G%Domain%mpp_domain)
-  ! deallocate(G%Domain)
+  deallocate(G%Domain%mpp_domain)
+  deallocate(G%Domain)
 
   deallocate(G)