DOC: Remove reference to sql fallback support for mysql connection objects

[cdcadman]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://github.com/pandas-dev/pandas/blob/main/doc/source/user_guide/io.rst#sql-queries

Documentation problem

The user guide states that mysql connections are still supported. Currently, pandas emits a warning if users try to pass a mysql connection to read_sql or DataFrame.to_sql (#45679)

Suggested fix for documentation

Currently, pandas emits a warning if users try to pass a mysql connection to read_sql or DataFrame.to_sql, so I would recommend changing the following paragraph in io.rst

If SQLAlchemy is not installed, a fallback is only provided for sqlite (and
for mysql for backwards compatibility, but this is deprecated and will be
removed in a future version).
This mode requires a Python database adapter which respect the `Python
DB-API <https://www.python.org/dev/peps/pep-0249/>`__.

to

If SQLAlchemy is not installed, you can use a `sqlite3.Connection` object instead.