Remove stale xfail on schema-qualified INSERT parseutils test

[pacocartones]

Description

test_simple_insert_single_table_schema_qualified in
tests/parseutils/test_parseutils.py was marked @pytest.mark.xfail back when an
older sqlparse mislabeled schema-qualified INSERT statements. That has long
been fixed. The test now passes across the whole supported range
(sqlparse >=0.3.0,<0.7 — I checked 0.3.0 and 0.6.0), so the marker only ever
produced an XPASS.

Since xfail_strict isn't set, that XPASS was silent, which means the assertion
wasn't actually guarding extract_tables anymore. This just drops the decorator
so the test does its job again. No production code changes.

Verified locally: the file passes repeatedly on both sqlparse 0.3.0 and 0.6.0,
and ruff format/ruff check are clean.

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected (this may include manually testing them in your local environment, or in other available environments). Cross this out if not relevant (for example, if you're making a documentation change).
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[dbaty]

Please remove this. The changelog is for users of pgcli, who are unlikely to feel concerned and interested by this change.

Also, I am not sure that this warrants an addition in AUTHORS. Unless you really want your name there, of course...

[dbaty]

Also, there is another xfail on tests.test_sqlcompletion.test_sub_select_multiple_col_name_completion(). It's been added by 4e86201 in 2015 (without explanation, unfortunately). Care to check if it's still needed? :)