directfb: remove BR2_PACKAGE_DIRECTFB_UNIQUE option

This option depends on BR2_UNIMPLEMENTED (which does not exist) since
2009. Since BR2_UNIMPLEMENTED does not exist, it was not possible to
enable BR2_PACKAGE_DIRECTFB_UNIQUE, which means that nobody cared
about making it available, and that we don't need to add any
Config.in.legacy handling to remove this option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>