The following reference packs are removed with this PR.

microsoft.bcl.asyncinterfaces/1.0.0
microsoft.bcl.asyncinterfaces/1.1.0
microsoft.build.framework/15.1.548
microsoft.build.framework/16.6.0
microsoft.build.tasks.core/15.1.1012
microsoft.build.tasks.core/15.1.548
microsoft.build.utilities.core/15.1.548
microsoft.build.utilities.core/16.6.0
microsoft.build/16.6.0
microsoft.csharp/4.7.0
microsoft.dotnet.platformabstractions/2.0.0
microsoft.extensions.dependencyinjection.abstractions/5.0.0
microsoft.extensions.dependencyinjection/5.0.0
microsoft.extensions.dependencymodel/2.1.0
microsoft.extensions.filesystemglobbing/5.0.0
microsoft.extensions.logging.abstractions/5.0.0
microsoft.extensions.logging/5.0.0
microsoft.extensions.options/5.0.0
microsoft.extensions.primitives/5.0.0
nuget.build.tasks/5.1.0
nuget.commands/5.1.0
nuget.common/5.1.0
nuget.configuration/5.1.0
nuget.credentials/5.1.0
nuget.dependencyresolver.core/5.1.0
nuget.frameworks/5.1.0
nuget.librarymodel/5.1.0
nuget.packaging/5.1.0
nuget.projectmodel/5.1.0
nuget.protocol/5.1.0
nuget.versioning/4.3.0
nuget.versioning/4.4.0
nuget.versioning/5.1.0
system.buffers/4.0.0
system.collections.immutable/1.4.0
system.collections.immutable/1.6.0
system.collections.immutable/1.7.1
system.collections.specialized/4.0.1
system.componentmodel.eventbasedasync/4.0.11
system.componentmodel.primitives/4.1.0
system.componentmodel.typeconverter/4.1.0
system.componentmodel/4.0.1
system.composition.attributedmodel/1.0.31
system.composition.attributedmodel/1.3.0
system.composition.convention/1.0.31
system.composition.convention/1.3.0
system.composition.hosting/1.0.31
system.composition.hosting/1.3.0
system.composition.runtime/1.0.31
system.composition.runtime/1.3.0
system.composition.typedparts/1.0.31
system.composition.typedparts/1.3.0
system.data.sqlclient/4.8.2
system.diagnostics.contracts/4.0.1
system.diagnostics.contracts/4.3.0
system.diagnostics.diagnosticsource/5.0.0
system.diagnostics.fileversioninfo/4.0.0
system.diagnostics.textwritertracelistener/4.0.0
system.io.compression.zipfile/4.0.1
system.io.pipelines/4.7.0
system.io.pipelines/5.0.1
system.io.pipes.accesscontrol/5.0.0
system.memory/4.5.0
system.memory/4.5.1
system.memory/4.5.2
system.net.http/4.3.2
system.net.http/4.3.3
system.net.sockets/4.1.0
system.reflection.metadata/1.5.0
system.reflection.metadata/1.7.0
system.reflection.metadata/1.8.1
system.runtime.compilerservices.unsafe/4.5.0
system.runtime.compilerservices.unsafe/4.6.0
system.runtime.compilerservices.unsafe/4.7.1
system.runtime.serialization.json/4.0.2
system.security.cryptography.cng/4.4.0
system.security.cryptography.cng/4.5.0
system.security.cryptography.openssl/4.5.0
system.security.cryptography.openssl/4.5.1
system.security.cryptography.pkcs/4.5.0
system.security.cryptography.pkcs/4.5.2
system.text.encoding.codepages/4.3.0
system.text.encoding.codepages/4.5.1
system.text.encodings.web/4.6.0
system.text.encodings.web/4.7.0
system.text.encodings.web/5.0.0
system.text.json/4.6.0
system.text.json/4.7.0
system.text.json/4.7.1
system.text.json/5.0.0
system.threading.tasks.extensions/4.5.1
system.threading.tasks.extensions/4.5.2
system.threading.tasks.extensions/4.5.3
system.xml.xpath.xdocument/4.3.0
system.xml.xpath/4.0.1
system.xml.xpath/4.3.0

The full list of removed files can be viewed at https://gist.github.com/MichaelSimons/2ae873b6a46148a2168a88d6859a4af2