Drop building 4.16 pre1 and pre2

[philipzae]

I think you can disable the building of ubuntu_20.04-xfce-4.16pre2 and ubuntu_20.04-xfce-4.16pre1, as their builds don't actually change since the first time they were built and maybe consider building the 4.16 final release.

[schuellerf]

Dropped those...