cmd/compile/internal/gc: remove unused exporter.nesting field

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>

src/cmd/compile/internal/gc/bexport.go

@@ -183,9 +183,6 @@ type exporter struct {
 	written int // bytes written
 	indent  int // for p.trace
 	trace   bool
-
-	// work-around for issue #16369 only
-	nesting int // amount of "nesting" of interface types
 }
 
 // export writes the exportlist for localpkg to out and returns the number of bytes written.