Remove unused disable_identity_server config flag

Fixes https://github.com/vector-im/riot-web/issues/10229

config.sample.json

@@ -8,7 +8,6 @@
             "base_url": "https://vector.im"
         }
     },
-    "disable_identity_server": false,
     "disable_custom_urls": false,
     "disable_guests": false,
     "disable_login_language_selector": false,

docs/config.md

@@ -20,10 +20,10 @@ For a good example, see https://riot.im/develop/config.json.
      if `default_hs_url` is used.
    * The identity server is used for verifying third party identifiers like emails
      and phone numbers. It is not used to store your password or account information.
-     If not provided, the identity server defaults to vector.im unless `disable_identity_server`
-     is set to true in the config. Currently the only two public identity servers
-     are https://matrix.org and https://vector.im, however in future identity servers
-     will be decentralised.
+     If not provided, the identity server defaults to vector.im. Currently the only 
+     two public identity servers are https://matrix.org and https://vector.im, however 
+     in future identity servers will be decentralised. In the future it will be possible
+     to disable the identity server functionality.
    * Riot will fail to load if a mix of `default_server_config`, `default_server_name`, or
      `default_hs_url` is specified. When multiple sources are specified, it is unclear
      which should take priority and therefore the application cannot continue.

test/app-tests/joining.js

@@ -104,7 +104,6 @@ describe('joining a room', function() {
                     hsName: "TEST_ENVIRONMENT",
                     hsNameIsDifferent: false, // yes, we lie
                     isUrl: IS_URL,
-                    identityEnabled: true,
                 }),
             };
 

test/app-tests/loading.js

@@ -153,7 +153,6 @@ describe('loading:', function() {
                 hsName: "TEST_ENVIRONMENT",
                 hsNameIsDifferent: false, // yes, we lie
                 isUrl: DEFAULT_IS_URL,
-                identityEnabled: true,
             }),
             embeddedPages: {
                 homeUrl: 'data:text/html;charset=utf-8;base64,PGh0bWw+PC9odG1sPg==',