Remove duplicate systemd parameters from kernel-mshv cfg

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Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• Packages depending on static components modified in this PR (Golang, *-static subpackages, etc.) have had their Release tag incremented.
• Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json, .github/workflows/cgmanifest.json)
• LICENSE-MAP files are up-to-date (./SPECS/LICENSES-AND-NOTICES/data/licenses.json, ./SPECS/LICENSES-AND-NOTICES/LICENSES-MAP.md, ./SPECS/LICENSES-AND-NOTICES/LICENSE-EXCEPTIONS.PHOTON)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

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Summary

What does the PR accomplish, why was it needed?
When booting, grub should still be getting its systemd-related parameters from one place: systemd.cfg which is packaged in systemd.rpm. Prevent the kernel-mshv package from stomping over these settings.

Change Log

• Remove "net.ifnames=0 plymouth.enable=0 systemd.legacy_systemd_cgroup_controller=yes systemd.unified_cgroup_hierarchy=0" from $kernel_cmdline_mshv

Does this affect the toolchain?

NO

Associated issues

• https://microsoft.visualstudio.com/OS/_workitems/edit/45063749/

Test Methodology

• Buddy building and installing the resulting rpm artifact to ensure no regressions.
• https://dev.azure.com/mariner-org/mariner/_build/results?buildId=382761&view=results

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Tested locally, works!