treewide: remove redundant numprocesses pytest-xdist parameter

Including the pytest-xdist package automatically configures numprocesses
to $NIX_BUILD_CORES, so specifying these on a package level has become
redundant.

pkgs/development/python-modules/datashader/default.nix

@@ -70,7 +70,6 @@ buildPythonPackage rec {
   '';
 
   pytestFlagsArray = [
-    "-n $NIX_BUILD_CORES"
     "datashader"
   ];