Cleanup tasks in SecurityManager/SetupEncryptionStore (#12764)

* Remove call to no-op `checkOwnCrossSigningTrust`

this is a no-op on rust crypto

* inline `SecurityManager.isCachingAllowed`

Since https://github.com/matrix-org/matrix-react-sdk/pull/4789, this has just
been an obscure way to write a test of a local variable.

* Remove unused `CreateSecretStorageOpts.getKeyBackupPassphrase` parameter

This is unused on rust crypto (cf https://github.com/matrix-org/matrix-js-sdk/pull/4313)

src/stores/SetupEncryptionStore.ts

@@ -152,8 +152,6 @@ export class SetupEncryptionStore extends EventEmitter {
             // in the background.
             await new Promise((resolve: (value?: unknown) => void, reject: (reason?: any) => void) => {
                 accessSecretStorage(async (): Promise<void> => {
-                    await cli.checkOwnCrossSigningTrust();
-
                     // The remaining tasks (device dehydration and restoring
                     // key backup) may take some time due to processing many
                     // to-device messages in the case of device dehydration, or