TensorFlow: remove dead code (tensorflow#1180)

Both the CMake and SPM builds now unconditionally enable the X10
backend.  As there is no configuration knob to control whether X10
backend support can be disabled, remove the now dead code path.

Author: compnerd

Package.swift

@@ -68,7 +68,6 @@ let package = Package(
         .product(name: "Numerics", package: "swift-numerics"),
       ],
       swiftSettings: [
-        .define("USING_X10_BACKEND"),
         .define("DEFAULT_BACKEND_EAGER"),
       ]),
     .target(