Revert "Add GCC9 to Krita AppImage base image"

GCC9 cannot be used due to various incompatibilities, so lets remove it.

This reverts commit 69dd4c6a.
2 jobs for master
Status Name Job ID Coverage
  Build
manual kdeorg/appimage-base #129679
docker allowed to fail manual
manual kdeorg/staticweb #129680
docker allowed to fail manual