ipn/store: remove a layer of indirection for registering stores (#15986)

Registering a new store is cheap, it just adds a map entry. No need to
lazy-init it with sync.Once and an intermediate slice holding init
functions.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/store/store_aws.go

@@ -12,10 +12,6 @@ import (
 )
 
 func init() {
-	registerAvailableExternalStores = append(registerAvailableExternalStores, registerAWSStore)
-}
-
-func registerAWSStore() {
 	Register("arn:", func(logf logger.Logf, arg string) (ipn.StateStore, error) {
 		ssmARN, opts, err := awsstore.ParseARNAndOpts(arg)
 		if err != nil {