[KeyringController] Drop uncached encryption support

[mikesposito]

The KeyringController currently supports both cached and uncached encryption. This is a legacy feature that is no longer needed, as all clients have this enabled by default.

Specifically, when cacheEncryptionKey is set to false, the KeyringController will not cache the encryption key, which is the default behavior in all current clients. Removing this feature simplifies the codebase and reduces complexity of critical parts of code responsible of keeping the vault secure.