MNT: Avoid some UnitStrippedWarnings

Avoid passing Quantity instances to matplotlib plots that don't support
them in the declarative interface.

src/metpy/plots/declarative.py

@@ -1064,7 +1064,8 @@ def _build(self):
         kwargs.setdefault('colors', self.linecolor)
         kwargs.setdefault('linestyles', self.linestyle)
 
-        self.handle = self.parent.ax.contour(x_like, y_like, imdata, self.contours, **kwargs)
+        self.handle = self.parent.ax.contour(x_like, y_like, imdata.metpy.dequantify(),
+                                             self.contours, **kwargs)
         if self.clabels:
             self.handle.clabel(inline=1, fmt='%.0f', inline_spacing=8,
                                use_clabeltext=True, fontsize=self.label_fontsize)
@@ -1205,11 +1206,13 @@ def griddata(self):
 
             if self.plot_units is not None:
                 data_subset_u = data_subset_u.metpy.convert_units(self.plot_units)
+                data_subset_u = data_subset_u.metpy.dequantify()
                 data_subset_v = data_subset_v.metpy.convert_units(self.plot_units)
+                data_subset_v = data_subset_v.metpy.dequantify()
             self._griddata_u = data_subset_u * self.scale
             self._griddata_v = data_subset_v * self.scale
 
-        return (self._griddata_u, self._griddata_v)
+        return self._griddata_u, self._griddata_v
 
     @property
     def plotdata(self):