Remove unused xunit.abstractions.snk #1488
Description
xunit.abstractions was removed in https://github.com/dotnet/source-build-externals/pull/472/files but its key was not. This key should be removed as it is no longer referenced.
Note: the removal happened in https://github.com/dotnet/source-build-externals prior to merging the repo into https://github.com/dotnet/source-build-reference-packages but they key was transferred during the repo consolidation