Documentation: features: remove c6x references

The references to arch/c6x are obsolete now that the architecture
is gone. Remove them.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210225142841.3385428-1-arnd@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>