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/build/default.nix

@@ -52,8 +52,6 @@ buildPythonPackage rec {
   ];
 
   pytestFlagsArray = [
-    "-n"
-    "$NIX_BUILD_CORES"
     "-W"
     "ignore::DeprecationWarning"
   ];