doc: the GN files should use Node's license

This is a followup to https://github.com/nodejs/node/pull/47637#issuecomment-1806879616.

Remove the copyright headers in the GN files since they should just be licensed under Node's license file. (Those files were written by Electron team from scratch up.)

The gypi_to_gn.py script comes from Chromium and an entry for it has been added in license-builder.sh.