ext: Avoid specifying empty interfaces and embrace duck typing

ext: Avoid specifying empty interfaces and embrace duck typing

It turns out no handler is implementing

prehandle()
posthandle()

JIRA: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-533

Change-Id: Ie8d92027f29fc33192fcf0d495fd3c4f6e4075aa
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30217
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>