python: Remove unnecessary exports from pybind enums

python: Remove unnecessary exports from pybind enums

According to pybind documentation [1], enum entries use
.export_values() to export the enum entries into the parent
scope. However, strongly typed C++11 class enums are in their own
scope and therefore do not need to be exported.

[1]: https://pybind11.readthedocs.io/en/stable/classes.html#enume
rations-and-internal-types

Change-Id: I6181306b530d59eaedcb3daf9cab0a03d01d56f4
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25709
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>