go/token: clear cache after grabbing the mutex in RemoveFile
This fixes a race, that was possible to hit in RemoveFile.
The file cache could have been populated concurrently just before
grabbing of the mutex.
Change-Id: I6a6a696452d8bd79ac4ac6574297390978353444
Reviewed-on: https://go-review.googlesource.com/c/go/+/705756
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>