Drop index on key metadata table

Key metadata can exist without a keyring in the table, so we don't lose
it on reinsertion.

Also, don't use transactions inside database upgrade tasks. The entire
update is wrapped in a transaction, and we don't want to break this!

OpenKeychain/src/main/sqldelight/org/sufficientlysecure/keychain/KeyMetadata.sq

@@ -1,11 +1,12 @@
 import java.lang.Boolean;
 import java.util.Date;
 
+-- NOTE: This table's data is NOT bound to keyrings_public! There may be
+-- entries here that no longer have a corresponding key in the database!
 CREATE TABLE IF NOT EXISTS key_metadata (
     master_key_id INTEGER PRIMARY KEY,
     last_updated INTEGER AS Date,
-    seen_on_keyservers INTEGER AS Boolean,
-    FOREIGN KEY(master_key_id) REFERENCES keyrings_public(master_key_id) ON DELETE CASCADE
+    seen_on_keyservers INTEGER AS Boolean
 );
 
 selectByMasterKeyId: