exporter.nesting was added in c7b9bd74 to mitigate #16369 which was
closed in ee272bbf. Remove the exporter.nesting field as it is now unused.

Change-Id: I07873d1a07d6a08b11994b817a1483ffc2f5e45f
Reviewed-on: https://go-review.googlesource.com/38490
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>