core: drop dead Crypto::enabled() left by the base64 fix

The signed-out base64 fix (61f4fbe) made open_b64 unconditional, removing the
only caller of Crypto::enabled(). Delete the now-dead method to keep the build
warning-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/crypto.rs

@@ -30,10 +30,6 @@ impl Crypto {
         }
     }
 
-    pub(crate) fn enabled(&self) -> bool {
-        self.cipher.is_some()
-    }
-
     /// nonce(12) ‖ ciphertext. Passthrough when disabled.
     pub(crate) fn seal(&self, plaintext: &[u8]) -> Vec<u8> {
         match &self.cipher {