doc: Remove oudated documentation about gnulib updates (bug #57044) * etc/GNULIB.UPDATE: Remove file with oudated instructions. The current instructions about the gnulib update are given in `bootstrap.conf` and reduce to replacing a singe commit id and running `./bootstrap --bootstrap-sync`, see cset ece72b94486f.

root = true

[*]
charset = utf-8
end_of_line = lf
indent_size = 2
indent_style = space
insert_final_newline = true

[*.{am,mk}]
indent_size = tab
indent_style = tab