Remove installation of settings.ini

[ryonakano]

This file is removed in elementary/default-settings#305 and causes the latest build failing:

========================================================================
Building module default-gtk-settings in /home/runner/work/flatpak-platform/flatpak-platform/.flatpak-builder/build/default-gtk-settings-1
========================================================================
Already on 'master'
Running: install -Dm644 gtk/settings.ini /usr/etc/gtk-3.0/settings.ini
install: cannot stat 'gtk/settings.ini': No such file or directory
Error: module default-gtk-settings: Child process exited with code 1
Error: Process completed with exit code 1.

https://github.com/elementary/flatpak-platform/actions/runs/8906581018/job/24458972139