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)