libotutil: Drop accidental use of GLib 2.36 API

Sticking with 2.34 for a while longer.
Revert "libotutil: Make use of GBytes in ot_variant_read()"

This reverts commit c77908bf514d61e75798932f61b5b414d9e36a3c; we can't
do this since g_variant_get_data_as_bytes() is a GLib 2.36 API.