dts: drop dangling c6x symlink

With c6x architecture removal, scripts/dtc/include-prefixes/c6x symlink
lost its target. Drop the dangling symlink which triggers some distribution
check scripts.

Fixes: a579fcfa

 ("c6x: remove architecture")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210223204114.E7F55E0155@unicorn.suse.cz