toolchain-external: remove SOURCE/SITE for locally available toolchain

toolchain-external: remove SOURCE/SITE for locally available toolchain

When the custom external toolchain is locally available, we currently
define SITE/SOURCE to empty variables. Now that the package
infrastructure doesn't define a value for SOURCE when VERSION is empty,
it doesn't attempt to download a file anymore, so we can get rid of
those empty SOURCE/SITE variables in the toolchain-external package.

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