boost: Do not force numpy support from 1.65

The reason is that if cross compiling (or for other reasons) python
bindings as a whole are turned off. Those two lines then trigger
assertion errors unless manually overridden for cross compilation.

This way:

 1. The `enableNumpy` default respects the `enablePython deafult.
 2. Cross works by default
 3. Absurd manual overrides still break as they should
 4. The `>= 1.65` logic is direct and not a maintaince gotcha.

pkgs/development/libraries/boost/1.65.nix

@@ -9,6 +9,4 @@ callPackage ./generic.nix (args // rec {
     sha256 = "9807a5d16566c57fd74fb522764e0b134a8bbe6b6e8967b83afefd30dcd3be81";
   };
 
-  enableNumpy = args.enableNumpy or true;
-
 })