README: drop obsolete note about `//export` declarations (#615)

Since #550 in June 2022, we have stopped considering `//export`
directives when deciding what names to obfuscate.
We forgot to remove the caveat from the README.

README.md

@@ -140,8 +140,6 @@ to document the current shortcomings of this tool.
    var _ = reflect.TypeOf(Message{})
    ```
 
-* Go declarations exported for cgo via `//export` are not obfuscated.
-
 * Go plugins are not currently supported; see [#87](https://github.com/burrowers/garble/issues/87).
 
 ### Contributing