Remove empty Debian package python3-glue-ligolw-tools

The package became empty in commit af32be70, we simply forgot to remove the package from debian/control.

It is the convention on Debian that when a Python package provides executables installed in bin, binaries are installed for only one Python version (the theory being that the user only cares about Python 2 or 3 when they have to import a module, not when they run a script from the command line).

For review by @ryan.fisher.