Remove postinstall-build

[fulv]

Running make all produces this warning:

npm WARN deprecated [email protected]: postinstall-build's behavior is now built into npm! You should migrate off of postinstall-build and use the new `prepare` lifecycle scri
pt with npm 5.0.0 or greater.                                                                                                                                                        

As far as I can see, postinstall-build is not being used anywhere.

Can we get rid of it?