startplasma-wayland: Don't set GDK_SCALE and GDK_DPI_SCALE

We did this to work around a bug whereby envars would be inappropriately
propagated when going from X11 to Wayland, causing GTK apps to be scaled
to too large a size

However this no longer appears to be an issue with any of the GTK apps I
tested (Blanket, Bustle, Cheese, Chromium, Firefox, Gedit, GIMP, GNOME
Sudoku, Inkscape, Peek, Video Downloader), and keeping it causes a bug
whereby non-GTK XWayland apps that perhaps inappropriately look at
these envars to determine their scale are scaled too small.

This hack has outlived its usefulness; let's kill it.

BUG: 443215
FIXED-IN: 5.26.3


(cherry picked from commit 63de3d6f)