Update airs.MD to remove reference to naccess

[amjjbonvin]

The airs.md file describes the restraints generation from chemical shift perturbation using rather outdated commands.

It would be best to remove reference to naccess and use instead the haddock-restraints software to generate the restraints.

[VGPReys]

OK , on it

[VGPReys]

Ow, actually, the file airs.md is not used in the documentation, but just sitting there in the repo...
Let me think of a way to get the relevant information from it to some other place

[amjjbonvin]

Or we simply delete it from the repo. We might instead write a tutorial on generating various types of restraints

[VGPReys]

Yes, I am removing it.