fix(commons): drop unnecessary non-null assertion in vaultDelete

K2 propagates the `contents != null` implication carried by the
`removedFromVault` local val, so the `!!` inside the `if (removedFromVault)`
branch is redundant and the compiler warns on it. The `encodeVault(contents)`
call in the same branch already relied on that smart cast.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D93heBmNF3dPZwNmBHbH8k

commons/src/jvmMain/kotlin/com/vitorpamplona/amethyst/commons/keystorage/SecureKeyStorage.kt

@@ -441,7 +441,7 @@ actual class SecureKeyStorage private actual constructor() {
             }
 
         if (removedFromVault) {
-            if (contents!!.isEmpty()) {
+            if (contents.isEmpty()) {
                 try {
                     keyring().deletePassword(SERVICE_NAME, vaultAlias)
                 } catch (_: PasswordAccessException) {