Remove unnecessary package references

[pranavkm]

We should do a scan and figure out if there are any package references we don't actually need in our projects. For instance, aspnet/Logging#585

[pranavkm]

cc @davidfowl

[Eilon]

In theory the work is this:

1. Find all csproj files in all src folders in all repos
2. Delete all "System.*" package references
3. If it compiles, send a PR and get it merged
4. If it doesn't compile, figure out what's needed and go to step (3)

[pakrym]

I was thinking about checking which package dll's got optimized out from assembly references by compiler.

[Eilon]

@pakrym that could certainly work as well.