Remove runtimes sections from projects

No longer necessary to restore/build. Removing them allows NuGet to
restore only the packages necessary for the current platform, vastly
reducing cache sizes and restore time.

My `~/.nuget/packages` directory was almost 1GB smaller with this
change.

src/Microsoft.PowerShell.Linux.Host/project.json

@@ -25,14 +25,5 @@
         "netstandardapp1.5": {
             "imports": [ "dnxcore50", "portable-net45+win8" ]
         }
-    },
-
-    "runtimes": {
-        "ubuntu.14.04-x64": { },
-        "centos.7.1-x64": { },
-        "win7-x64": { },
-        "win10-x64": { },
-        "osx.10.10-x64": { },
-        "osx.10.11-x64": { }
     }
 }

src/TypeCatalogParser/project.json

@@ -13,14 +13,5 @@
         "netstandardapp1.5": {
             "imports": [ "dnxcore50", "portable-net45+win8" ]
         }
-    },
-
-    "runtimes": {
-        "ubuntu.14.04-x64": { },
-        "centos.7.1-x64": { },
-        "win7-x64": { },
-        "win10-x64": { },
-        "osx.10.10-x64": { },
-        "osx.10.11-x64": { }
     }
 }