Remove symlinks in repo

[opcode81]

Two files in the repo are symlinks:

• examples/vizdoom/network.py -> ../atari/atari_network.py
• examples/modelbased/psrl.py -> ../../test/modelbased/test_psrl.py

The use of symlinks can cause problems on Windows (git setting core.symlinks must be enabled and Windows Developer Mode must be used). Unless there is a very good reason to have symlinks, they should be removed.

[MischaPanch]

The symlinks are there for executing examples inside tests. I have the feeling that this mechanism should generally be adjusted, after which this issue may become obsolete. I'm putting it on hold for now, as it doesn't seem to be very urgent (given the limited number of devs on windows)