Deprecate gateway mode

[agazso]

Task

There is an undocumented gateway mode (gateway-mode config variable) which disables certain endpoints. With the Restricted API and the gateway-proxy released the gateway mode is no longer need to be in Bee. Once the Restricted API is no longer experimental and the gateway-proxy is documented the gateway mode can be removed. In the meanwhile the deprecation process can be started.

Acceptance criterea

The gateway-mode config variable is removed and all the code that forbids endpoints from being accessed is also removed.