Remove <Imports> now covered by Directory.Build.props.

[jpobst]

In #605, we created a Directory.Build.props file which automatically imports several things for all projects. The projects that are still manually importing these same files are creating duplicate warnings like:

tests\Java.Interop-Tests\Java.Interop-Tests.csproj(33,3): 
Warning MSB4011: "d:\a\1\s\bin\BuildRelease\JdkInfo.props" cannot be imported again. It was already imported at "d:\a\1\s\Directory.Build.props (13,3)". This is most likely a build authoring error. This subsequent import will be ignored. 

This doesn't fix the jnienv-gen.csproj warnings, they will be fixed when the project is converted to sdk-style.