xtensa: Remove arch-specific (and empty) offsets.h

The xtensa arch code had this empty offsets.h header sitting around.
Its name collides with the autogenerated offsets.h, making it
dangerously dependent on include file path order.  Seems to be benign,
but it's freaking me out.  Remove.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>