wayland: Remove GNOME Classic-specific workaround

As GSettings now supports session-specific defaults, GNOME Classic
no longer uses a separate schema and the decoration layout is always
determined by the regular schema.

This reverts commit add67b51.

!401
2 jobs for wip/fmuellner/deco-setting-3 in 16 minutes and 29 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #137797
build

00:16:29

manual #137798
win32 allowed to fail manual
msys2-mingw32