[gateway] Remove policy reload, instead read policy from backend (#7727)

Inconsistencies can arise after applying bucket policies in
gateway mode, since all gateway instances do not share a
common shared state. This is by design to keep gateway as
shared nothing architecture.

This PR fixes such inconsistencies by reloading policy
if any from the backend.

Fixes #7723

cmd/gateway-main.go

@@ -304,6 +304,9 @@ func StartGateway(ctx *cli.Context, gw Gateway) {
 		printGatewayStartupMessage(getAPIEndpoints(), gatewayName)
 	}
 
+	// Set when gateway is enabled
+	globalIsGateway = true
+
 	// Set uptime time after object layer has initialized.
 	globalBootTime = UTCNow()