xburst: Remove n516, n526 and id800wt subtargets

The driver code for supporting these subtargets was already removed a while ago,
so
the current images wont actually boot.

It is also quite unlikely that there are any users of them at this point, so
remove them.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

SVN-Revision: 44586