Explicitly remove grids for pcolormesh (#7219)

Author: Illviljan

xarray/plot/dataarray_plot.py

@@ -2318,6 +2318,7 @@ def pcolormesh(
             y = _infer_interval_breaks(y, axis=1, scale=yscale)
             y = _infer_interval_breaks(y, axis=0, scale=yscale)
 
+    ax.grid(False)
     primitive = ax.pcolormesh(x, y, z, **kwargs)
 
     # by default, pcolormesh picks "round" values for bounds