software/theia: Fix logo.png installation

In older theia versions `~/srv/frontend-static/logo.png` was a symlink
to the actual logo file. Now the logo is copied into `frontend-static`
but a simple `cp -f` does not behave as expected when the destination
already exists and is a symbolic link:
- if the symlink is valid, the file is copied to the symlink target
- if the symlink is broken, the copy command fails

So the destination must be removed before copying.