Remove redundant explain_todo for clientError SYNTAX_ERROR stmt (#118)

Test 03254_test_alter_user_no_changes has stmt2 with clientError
SYNTAX_ERROR which means ClickHouse expects it to fail parsing.
There's no explain_2.txt since ClickHouse can't produce EXPLAIN
output for a syntax error. Our parser is more permissive and
parses it anyway, but there's nothing to compare against.

Removing stmt2 from explain_todo since:
- No explain_2.txt exists to compare against
- Test already skips due to missing file
- Parser being more permissive is acceptable behavior

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

parser/testdata/03254_test_alter_user_no_changes/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt2": true
-  }
-}
+{}