Apply suggestions from code review

Co-authored-by: Joshua Lampert <51029046+JoshuaLampert@users.noreply.github.com>

Author: cwittens

src/equations/serre_green_naghdi_1d.jl

@@ -119,11 +119,8 @@ function initial_condition_manufactured(x, t,
 
     if equations.bathymetry_type isa BathymetryFlat
         D = zero(h)
-    elseif equations.bathymetry_type isa BathymetryVariable ||
-           equations.bathymetry_type isa BathymetryMildSlope
-        D = -(1.5 - cospi(2 * x))
     else
-        error("$(equations.bathymetry_type) not supported for initial_condition_manufactured for the Serre Green Naghdi Equation.")
+        D = -(1.5 - cospi(2 * x))
     end
 
     b = -D

test/test_serre_green_naghdi_1d.jl

@@ -325,7 +325,7 @@ end
                         cons_error=[1.7763568394002505e-15 0.00019895120429092028 0.0],
                         change_waterheight=-1.7763568394002505e-15,
                         change_entropy_modified=135.16210732695845,
-                        atol=1e-9) # to make CI pass)
+                        atol=1e-9) # to make CI pass
 
     @test_allocations(semi, sol, allocs=410_000)
 end