Remove gsettings-schema-convert tool

Remove gsettings-schema-convert tool

Having this tool in GLib is a bad idea for a number of reasons:

  - experience has shown that the simple file format was a bad idea

  - the tool is currently implemented with a hack that would require a
    dependency inversion to solve (the tool needs to depend on Python
    GVariant bindings)

  - the tool itself is unmaintained

It will be moved to the GConf git repository so people can continue to
use it for the purpose of converting GConf schemas.