doc: Remove obsolete README.440-DDR-performance file

The PPC 440 support has been removed in commit 98f705c9cefd
("powerpc: remove 4xx support") already, so this file is
certainly not required anymore.

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