The bug 38816 changed the lock that protects THD::query from
LOCK_thread_count to LOCK_thd_data, but didn't update the associated
InnoDB functions.

1. The innobase_mysql_prepare_print_arbitrary_thd and the
innobase_mysql_end_print_arbitrary_thd InnoDB functions have been
removed, since now we have a per-thread mutex: now we don't need to wrap
several inter-thread access tries to THD::query with a single global
LOCK_thread_count lock, so we can simplify the code.

2. The innobase_mysql_print_thd function has been modified to lock
LOCK_thd_data in direct way.