toolchain-crosstool-ng: remove support

In order to avoid the work of converting the toolchain-crosstool-ng
logic to the package infrastructure, we remove it from Buildroot,
since it has been deprecated since quite some time.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>