pressure-vessel: Always disable gtk3-nocsd
This module interposes in front of g_object_get(), and is known to cause crashes, even in programs that use GObject but not GTK, such as either pressure-vessel-launch or pressure-vessel-launcher (it's unclear which).
Resolves: https://github.com/ValveSoftware/steam-runtime/issues/286
/cc @denittis
Edited by Simon McVittie