Revert changes to datasets/samples.py and helpers/testing.py

Cherry-picked to c2011d7 in #3922.

Author: weiji14

pygmt/datasets/samples.py

@@ -204,7 +204,7 @@ def _load_earth_relief_holes() -> xr.DataArray:
         is in meters.
     """
     fname = which("@earth_relief_20m_holes.grd", download="c")
-    return load_dataarray(fname, engine="gmt", decode_kind="grid")
+    return load_dataarray(fname, engine="netcdf4")
 
 
 class GMTSampleData(NamedTuple):

pygmt/helpers/testing.py

@@ -154,7 +154,7 @@ def load_static_earth_relief():
         A grid of Earth relief for internal tests.
     """
     fname = which("@static_earth_relief.nc", download="c")
-    return load_dataarray(fname, decode_kind="grid")
+    return load_dataarray(fname)
 
 
 def skip_if_no(package):