Remove stray use of old SWT_BUILDING_WITH_CMAKE condition. (#621)

We no longer define `SWT_BUILDING_WITH_CMAKE`, but one file's still
checking for it. Fix.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ABI/EntryPoints/ABIEntryPoint.swift

@@ -9,11 +9,7 @@
 //
 
 #if canImport(Foundation) && !SWT_NO_ABI_ENTRY_POINT
-#if SWT_BUILDING_WITH_CMAKE
-@_implementationOnly import _TestingInternals
-#else
 private import _TestingInternals
-#endif
 
 extension ABIv0 {
   /// The type of the entry point to the testing library used by tools that want