build: repair build after swiftlang/swift-tools-support-core#117

Remove Triple.swift as tools-support-core now has this defined.

```
/Users/buildnode/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-master/swiftpm/Sources/Build/BuildPlan.swift:1896:23: error: 'Triple' is ambiguous for type lookup in this context
fileprivate extension Triple.OS {
                      ^~~~~~
/Users/buildnode/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-master/swiftpm/.build/checkouts/swift-tools-support-core/Sources/TSCUtility/Triple.swift:21:15: note: found this candidate
public struct Triple: Encodable, Equatable {
              ^
/Users/buildnode/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-master/swiftpm/Sources/SPMBuildCore/Triple.swift:21:15: note: found this candidate
public struct Triple: Encodable, Equatable {
              ^
```

Author: compnerd

Sources/SPMBuildCore/CMakeLists.txt

@@ -11,8 +11,7 @@ add_library(SPMBuildCore
   BuildSystem.swift
   BuiltTestProduct.swift
   Sanitizers.swift
-  Toolchain.swift
-  Triple.swift)
+  Toolchain.swift)
 # NOTE(compnerd) workaround for CMake not setting up include flags yet
 set_target_properties(SPMBuildCore PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})