xtensa: remove s6000 variant and s6105 platform

The Stretch s6000 family support has been merged into mainline 5 years
ago. There appear to be no users of this code since nobody complained
that there is a merge error preventing compilation.

Apart from the s6105 IP camera reference design there are no s6000 devices
known to ever have run Linux and as the chips are out of production there
probably never will be. The successor s7000 no longer uses an Xtensa core
for the OS.

Let's remove the code until someone is found who actually needs it.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>