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)

test/test-utils/test-utils.ts

@@ -116,7 +116,6 @@ export function createTestClient(): MatrixClient {
         bootstrapCrossSigning: jest.fn(),
         hasSecretStorageKey: jest.fn(),
         getKeyBackupVersion: jest.fn(),
-        checkOwnCrossSigningTrust: jest.fn(),
 
         secretStorage: {
             get: jest.fn(),