Re-generating packages does not delete Customization.cs when the package is dropped as a result of removed TFMs

When TMS support is removed from SBRP and you regenerate packages, the Customization.cs file is not removed when the package no longer has any supported TFMs.  This is an edge case that was surfaced during the removal of netstandard 1.* (https://github.com/dotnet/source-build-reference-packages/pull/1179)