lzma: remove deprecated target package

The on-target lzma package has been deprecated for a long time, so remove
it. This does not remove the support for generating lzma-compressed rootfs
images.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>