README.md: stop mentioning Python

Python was needed by scripts/gen_*, which were removed.
Axe more LibNeubot-specific stuff

The src/neubot directory contained only stuff that I used to
run the current version of Neubot using `ctypes` to load code
from libneubot.so.

There is no need to keep this code in Libight.

As a side effect, src/ight_wrappers.cpp can now go as well.

Also, remove dead prototypes from src/ight_wrappers.h.