Remove now-obsolete flag from struct

This was changed to the more general `exechook`, but must have been
missed.

cmd/git-sync/main.go

@@ -276,7 +276,6 @@ type repoSync struct {
 	link       string         // the name of the symlink to publish under `root`
 	authURL    string         // a URL to re-fetch credentials, or ""
 	sparseFile string         // path to a sparse-checkout file
-	syncHookCmd string         // command to run after each sync
 	log        *logging.Logger
 	run        *cmd.Runner
 }
@@ -484,7 +483,6 @@ func main() {
 		link:       absLink,
 		authURL:    *flAskPassURL,
 		sparseFile: *flSparseCheckoutFile,
-		syncHookCmd: *flSyncHookCommand,
 		log:        log,
 		run:        cmdRunner,
 	}