fix: failing Ubi8-based Docker image build

[mattdibi]

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Brief description of the PR: This PR fixes the Ubi8-based Docker build of Kura

Description of the solution adopted: The package xmlstarlet was breaking the build because it was missing from the remote repo. To solve the issue it was removed from the required packages since it is not necessary anymore.

The xmlstarlet dependency was first introduced in https://github.com/eclipse/kura/pull/3778 with the sole purpose of disabling the Clock service in the starting snapshot (details here).

With https://github.com/eclipse/kura/pull/4178 the need of xmlstarlet was removed because we're relying on the docker_x86_64 installer which has its own starting snapshot for which the ClockService is already disabled (details here).

Therefore the xmlstarlet dependency can be safely removed.

[mattdibi]

Tested locally and I can correctly login to the Kura UI using the Ubi8-based image.