samsung-a3lte: move from testing to community (MR 1963)

There is already samsung-a3ulte in community, and the devices actually
share the same kernel, device tree, and basically everything except
firmware. We just need separate firmware packages because Samsung uses
different secure boot certificates on a3lte and a3ulte.

They also share the same wiki page which suggests installation using
the pre-built images now, so having pre-built images only for
samsung-a3ulte is quite confusing for users.

The situation is similar as for samsung-a5ulte and samsung-a5lte
which also are both in community already.