xtensa: Remove _xt_set_exception_handler()

This was a dead API.  Nothing ever used it, it wasn't exposed in any
API headers.  It never appeared in documentation.  It's not
particularly clear why a Zephy app would want to hook
architecture-specific exceptions instead of simply using the portable
error framework anyway. And it's not supported by asm2.  Delete.

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