Drop pdo_mysql_prepare_load_data.phpt

Like the test title and some comments in this test describe, this test
was supposed to have `::prepare()` failing because `LOAD DATA INFILE`
would not be supported as prepared statement, and then the test checks
whether follow-up queries would succeed.  However, `LOAD DATA INFILE`
is supported for prepared statements at least on Windows with mysqlnd,
so the test does no longer test what it is supposed to do.  Therefore,
we drop it.

Closes GH-6509.