dir.props: Remove obsolete UseRosylnCompilers
dir.props: Remove obsolete UseRosylnCompilers

Ref: https://github.com/Microsoft/msbuild/pull/483#issuecomment-184928288

sean-gilliam/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,cdmihai/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,jeffkl/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,mono/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,mono/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,rainersigwald/msbuild,chipitsine/msbuild,jeffkl/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,rainersigwald/msbuild,chipitsine/msbuild,mono/msbuild,rainersigwald/msbuild,Microsoft/msbuild,mono/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,chipitsine/msbuild