Drop -preview for 15.8 release (#3494)
Drop -preview for 15.8 release (#3494)


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