Remove LYNX KDI remainders

The last board that used to set CONFIG_LYNXKDI has been removed in
commit 242836a8 ("powerpc: ppc4xx: remove pcs440ep support"),
doc/README.lynxkdi only talks about a MPC8260 board being supported,
and the mpc8260 support has been removed four years ago in commit
2eb48ff7 ("powerpc, 8260: remove support for mpc8260") already,
and common/lynxkdi.c only consists of an "#error" statement these
days, so it seems like the LYNX KDI code is dead code nowadays.
Let's remove it now.

Signed-off-by: Thomas Huth <thuth@redhat.com>