Set DisableImplicitFrameworkReferences for all projects (dotnet/corefx#34786)

Set DisableImplicitFrameworkReferences for all projects (dotnet/corefx#34786)

* Set DisableImplicitFrameworkReferences for all projects

The SDK added targets which require the assets file when framework reference items are set.

We can disable the implicit FrameworkReference item in order to prevent these from running.

* Don't rely on implicit VB imports

DisableImplicitFrameworkReferences also doubles as DisableImplicitNamespaceImports.

Rather than enable this I think it's better just to stop relying on the default imports.

* Add missing reference to EventLog

Commit migrated from https://github.com/dotnet/corefx/commit/6653c5fe2adc7c7fca962df3fdc6675354726951