Private fields of the Go class are not used any more after the program
has exited. Delete them to allow JavaScript's garbage collection to
clean up the WebAssembly instance.

Updates #26193.

Change-Id: I349784a49eaad0c22ceedd4f859df97132775537
Reviewed-on: https://go-review.googlesource.com/122296
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>