[Diagnostics] Remove the C++ formatter for the "Swift" diagnostic style

We'll be using the new swift-syntax diagnostic formatter in the near
future, as it is nearly available on all host platforms. So, remove
the C++ formatter that did source-line annotation, falling back to the
"LLVM" style when swift-syntax is not compiled in.

include/swift/Basic/SourceManager.h

@@ -410,8 +410,6 @@ public:
                                SourceLoc();
   }
 
-  std::string getLineString(unsigned BufferID, unsigned LineNumber);
-
   /// Retrieve the buffer ID for \p Path, loading if necessary.
   unsigned getExternalSourceBufferID(StringRef Path);