speed-dreams: Remove from the channel

At 758 MiB compressed it's really too big for the channel.
ufoai: Remove from the channel

It's 898 MiB compressed.
xonotic: Remove from the channel

It's 878 MiB compressed.
alienarena: Remove from the channel

387 MiB compressed, 898 uncompressed.
vdrift: Remove from the channel

It's 564 MiB compressed.