gopls/internal/server: remove spurious Async in legacy RunGoVulncheck

In the partial rollback CL 627556, I failed to remove the call to
jsonrpc2.Async in RunGoVulncheck, which is now (once again) async to
request handling. This introduces a race, leading to a panic if the
handler responds before the call to Async.

Fixes golang/go#70355

Change-Id: I8b33599a3f62b82dc6acf9d9b3616194c0372b9c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/628236
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/server/command.go

@@ -1294,8 +1294,6 @@ func (c *commandHandler) RunGovulncheck(ctx context.Context, args command.Vulnch
 	}, func(ctx context.Context, deps commandDeps) error {
 		tokenChan <- deps.work.Token()
 
-		jsonrpc2.Async(ctx) // run this in parallel with other requests: vulncheck can be slow.
-
 		workDoneWriter := progress.NewWorkDoneWriter(ctx, deps.work)
 		dir := filepath.Dir(args.URI.Path())
 		pattern := args.Pattern