Baseline references in prebuilt error messages is confusing

[MichaelSimons]

Example build error:

##[error]eng/finish-source-only.proj(136,5): error : (NETCORE_ENGINEERING_TELEMETRY=Build) 1 new packages used not in baseline! See report at /__w/1/s/artifacts/log/Release/baseline-comparison.xml for more information. Package IDs are:
Microsoft.CodeAnalysis.CSharp.4.12.0
/__w/1/s/eng/finish-source-only.proj(136,5): error : 1 new packages used not in baseline! See report at /__w/1/s/artifacts/log/Release/baseline-comparison.xml for more information. Package IDs are:
/__w/1/s/eng/finish-source-only.proj(136,5): error : Microsoft.CodeAnalysis.CSharp.4.12.0
##[error]eng/finish-source-only.proj(136,5): error : (NETCORE_ENGINEERING_TELEMETRY=Build) Prebuilt usages are different from the baseline. If detected changes are acceptable, update baseline with:
cp '/__w/1/s/artifacts/log/Release/generated-new-baseline.xml' '/__w/1/s/eng/tools/prebuilt-baseline.xml'
/__w/1/s/eng/finish-source-only.proj(136,5): error : Prebuilt usages are different from the baseline. If detected changes are acceptable, update baseline with:
/__w/1/s/eng/finish-source-only.proj(136,5): error : cp '/__w/1/s/artifacts/log/Release/generated-new-baseline.xml' '/__w

We don't allow prebuilts anymore so updating the baseline is not acceptable. Perhaps the baseline concept could be removed now. We still have support for prebuilts via the prebuilts archive is for some reason they are needed for a point in time during the pre-preview time period where new TFMs are brought up.