directfb: remove broken X.org support

The BR2_PACKAGE_XSERVER option never existed, so it was never possible
for DirectFB to pass --enable-x11. Moreover, doing this without having
at least some dependency on certain X libraries will clearly not work.

Consequently, let's remove this bogus X.org support, and disable X11
support unconditionally (until someone is interested enough to bring
this back to life).

The option was actually named BR2_PACKAGE_DIRECTFB_XSERVER but was not
used by the .mk file.

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