Remove labs option to cache 'passphrase'

(which actually meant SSSS secrets)

Fixes https://github.com/vector-im/riot-web/issues/1392

src/CrossSigningManager.js

@@ -34,10 +34,7 @@ let secretStorageBeingAccessed = false;
 let passphraseOnlyOption = null;
 
 function isCachingAllowed() {
-    return (
-        secretStorageBeingAccessed ||
-        SettingsStore.getValue("keepSecretStoragePassphraseForSession")
-    );
+    return secretStorageBeingAccessed;
 }
 
 export class AccessCancelledError extends Error {

src/components/views/settings/tabs/user/LabsUserSettingsTab.js

@@ -66,7 +66,6 @@ export default class LabsUserSettingsTab extends React.Component {
                     <SettingsFlag name={"showHiddenEventsInTimeline"} level={SettingLevel.DEVICE} />
                     <SettingsFlag name={"lowBandwidth"} level={SettingLevel.DEVICE} />
                     <SettingsFlag name={"sendReadReceipts"} level={SettingLevel.ACCOUNT} />
-                    <SettingsFlag name={"keepSecretStoragePassphraseForSession"} level={SettingLevel.DEVICE} />
                 </div>
             </div>
         );

src/settings/Settings.js

@@ -521,11 +521,6 @@ export const SETTINGS = {
         displayName: _td("Enable message search in encrypted rooms"),
         default: true,
     },
-    "keepSecretStoragePassphraseForSession": {
-         supportedLevels: ['device', 'config'],
-         displayName: _td("Keep recovery passphrase in memory for this session"),
-         default: false,
-    },
     "crawlerSleepTime": {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
         displayName: _td("How fast should messages be downloaded."),