Just return true from wxString::Shrink()

The comparison of lengths() seems to be completely unnecessary.

Also document that Shrink() always returns true.

src/common/string.cpp

@@ -555,7 +555,7 @@ bool wxString::Shrink()
 {
   wxString tmp(begin(), end());
   swap(tmp);
-  return tmp.length() == length();
+  return true;
 }
 
 // deprecated compatibility code: