LynxOS is no longer supported

LynxOS needed the do_bootm_lynxkdi() function that got removed in
7e713067 ("Remove LYNX KDI remainders") - and that function needed
a lynxkdi_boot() function, where the last implementation had been
removed in 98f705c9 ("powerpc: remove 4xx support") already. Looks
like this OS is definitely not supported anymore, so remove it from
the corresponding lists.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Roese <sr@denx.de>