sparc: Pull the "Uknown" StaticInst class out of the ISA description.

sparc: Pull the "Uknown" StaticInst class out of the ISA description.

The only thing the Unknown format does is return an instance of this
class, so there's no reason to have it in the ISA description.

Change-Id: I3f8187b1450a8622a974c030c0cb552b26f6b5f0
Reviewed-on: https://gem5-review.googlesource.com/5463
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>