Remove the InstallDirRegKey

This should ensure that the default directory will always falls back to
`"$PROGRAMFILES64\${APP_NAME}"` where `APP_NAME` is:
`{{ app_name }} {{ version_major }}.{{ version_minor }}.{{ version_patch }}.{{ version_build }}`

Contributes to CURA-9459

packaging/NSIS/create_windows_installer.py

@@ -5,7 +5,6 @@ import argparse  # Command line arguments parsing and help.
 import subprocess
 
 import shutil
-import sys
 from datetime import datetime
 
 from pathlib import Path