util: Removed libelf-dev dep from Dockerfiles

util: Removed libelf-dev dep from Dockerfiles

The libelf-dev dependency is no longer required in our Dockerfiles.

This reverts commit 0cf67fb36281b17956d4dc10f05054bf711b4ba3,
https://gem5-review.googlesource.com/c/public/gem5/+/33596.

The libelf-dev dependency has been kept for the "all_dependencies"
Dockerfiles.

The corresponding Docker images have been built and uploaded to:
https://gcr.io/gem5-test.

Change-Id: Iacbd8240f69d476ad3a649baaccb6b85fec2487c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34676
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>