Remove non-functional swt_getWASIVersion(). (#1398)

A while back I implemented `swt_getWASIVersion()` to speculatively
return the value of `WASI_SDK_VERSION` as defined by WASI, but that
never landed and the [GitHub
issue](WebAssembly/wasi-libc#490) for it was
closed. Remove the inoperative helper function here.

### 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/Support/Versions.swift

@@ -89,9 +89,8 @@ let operatingSystemVersion: String = {
     }
   }
 #elseif os(WASI)
-  if let version = swt_getWASIVersion().flatMap(String.init(validatingCString:)) {
-    return version
-  }
+  // WASI does not have an API to get the current WASI or Wasm version.
+  // wasi-libc does have uname(3), but it's stubbed out.
 #else
 #warning("Platform-specific implementation missing: OS version unavailable")
 #endif

Sources/_TestingInternals/include/Versions.h

@@ -53,24 +53,6 @@ SWT_EXTERN void swt_getTestingLibraryCommit(const char *_Nullable *_Nonnull outH
 ///   testing library, or `nullptr` if that information is not available.
 SWT_EXTERN const char *_Nullable swt_getTargetTriple(void);
 
-#if defined(__wasi__)
-/// Get the version of the C standard library and runtime used by WASI, if
-/// available.
-///
-/// This function is provided because `WASI_SDK_VERSION` may or may not be
-/// defined and may or may not be a complex macro.
-///
-/// For more information about the `WASI_SDK_VERSION` macro, see
-/// [wasi-libc-#490](https://github.com/WebAssembly/wasi-libc/issues/490).
-static const char *_Nullable swt_getWASIVersion(void) {
-#if defined(WASI_SDK_VERSION)
-  return WASI_SDK_VERSION;
-#else
-  return 0;
-#endif
-}
-#endif
-
 SWT_ASSUME_NONNULL_END
 
 #endif