Swift: Remove positive Swift language tests

It's relatively complex to determine in advance if a Xcode, SDK,
and Deployment Target configuration is capable of running Swift.

For example the following combinations do not work:
 * deployment target < OS X 10.9
 * Xcode 6.2 and macosx10.9 SDK
 * Xcode 7 Beta 6 and macosx10.10 SDK

Until we found out how to query Xcode for Swift support in a reliable
way, the RunCMake.Swift test cases will be restricted to negative ones.

Tests/RunCMake/Swift/Enable-stdout.txt

@@ -1 +0,0 @@
--- The Swift compiler identification is Apple

Tests/RunCMake/Swift/Enable.cmake

@@ -1 +0,0 @@
-enable_language(Swift)

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -3,8 +3,6 @@ include(RunCMake)
 if(RunCMake_GENERATOR STREQUAL Xcode)
   if(XCODE_BELOW_6_1)
     run_cmake(XcodeTooOld)
-  else()
-    run_cmake(Enable)
   endif()
 else()
   run_cmake(NotSupported)