package/linux-headers: remove 2.6 snapshot

As Thomas states:
    It is completely weird to ask the tarball to be named exactly
    linux-2.6.tar.bz2. IMO, we should simply get rid of this feature.

Make it so! ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>