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.