Apply suggestions from code review (more changes to come)

* Updated KDE runtime to v5.15-21.08
* Removing old bits that shouldn't be there anymore.

Co-authored-by: Hubert Figuière <hub@figuiere.net>
Co-authored-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>

eu.ad5001.LogarithmPlotter.json

@@ -1,12 +1,12 @@
 {
     "app-id": "eu.ad5001.LogarithmPlotter",
     "runtime": "org.kde.Platform",
-    "runtime-version": "5.15",
+    "runtime-version": "5.15-21.08",
     "sdk": "org.kde.Sdk",
     "finish-args": [
         "--share=ipc",
         "--device=dri",
-        "--socket=x11",
+        "--socket=fallback-x11",
         "--socket=wayland",
         "--talk-name=org.freedesktop.DBus.Proprieties",
         "--talk-name=org.freedesktop.IBus",
@@ -17,7 +17,6 @@
     "rename-desktop-file": "logarithmplotter.desktop",
     "rename-icon": "logplotter",
     "rename-appdata-file": "eu.ad5001.LogarithmPlotter.metainfo.xml",
-    "cleanup": [ ],
     "post-install": [
         "install -D mimetypes /app/share/mime/packages/x-logarithm-plot.xml"
     ],