Remove legacy images

Author: ubruhin

README.md

@@ -26,16 +26,6 @@ In addition, this image contains
 [`linuxdeployqt`](https://github.com/probonopd/linuxdeployqt) to build the
 AppImage.
 
-### `ubuntu-20.04`
-
-Based on Ubuntu 20.04, containing Qt from the official Ubuntu package
-repository. This image is intended to check if LibrePCB compiles on a standard
-Ubuntu 20.04.
-
-In addition, this image contains necessary tools for dynamic linking of
-LibrePCB (pkg-config, libdxflib, libmuparser, libquazip5, libpolyclipping,
-googletest).
-
 ### `ubuntu-22.04`
 
 Based on Ubuntu 22.04, containing Qt from the official Ubuntu package