Removed top-level solution
Removed top-level solution

Core, mobile and portable should instead be built from their respective
solution files. This avoids problems with nuget package locations
getting messed up between different solutions.

kentcb/BrightstarDB,BrightstarDB/BrightstarDB,dharmatech/BrightstarDB,BrightstarDB/BrightstarDB,kentcb/BrightstarDB,kentcb/BrightstarDB,BrightstarDB/BrightstarDB,BrightstarDB/BrightstarDB,dharmatech/BrightstarDB,kentcb/BrightstarDB,dharmatech/BrightstarDB,dharmatech/BrightstarDB,dharmatech/BrightstarDB,dharmatech/BrightstarDB