Remove `ClearPreviousSBRP` target from `repo-projects/source-build-reference-packages.proj`

[NikolaMilosavljevic]

Publishing changes in arcade (dotnet/arcade#14559) are replacing old model of propagating artifacts from repo build to shared VMR locations.

Remove ClearPreviousSBRP target once the arcade changes have flown to installer repo.

New publishing model also adds a new task for clearing previous SBRPs during inner-build - after building and before publishing.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[NikolaMilosavljevic]

Blocked by dotnet/installer#19145