singleflight: remove forgotten field
singleflight: remove forgotten field

Port from CL 433315.

Change-Id: Iaf6ab4676dd10a73443c3c9981ffc77233b4631c
Reviewed-on: https://go-review.googlesource.com/c/sync/+/433555
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Auto-Submit: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/sync