python: Stop conditionally excluding code from pyobject.cc

python: Stop conditionally excluding code from pyobject.cc

Now that the Ether* classes are included in all builds, there's no
reason to conditionally compile code in pyobject.cc.

Change-Id: If94602af71774b1f090a3344a633207f4b37d308
Reviewed-on: https://gem5-review.googlesource.com/c/13470
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>