gstregistry: Don't look for plugins under 'debug' anymore

Submitted by Nirbheek Chauhan

Description

Originally, this was used in 2014 when we had the vcxproj files:

https://bugzilla.gnome.org/show_bug.cgi?id=679115

but don't create this directory structure with the meson build files,
so this just leads to plugins not being found if GStreamer is built
with the debug CRT.