Removed redundant plugin_thdvar_cleanup() from end_connection(): called by
THD::free_connection(), which always follows end_connection().

Saves at least one lock(LOCK_plugin) and one
rdlock(LOCK_system_variables_hash).

Benchmarked on a 2socket/20core/40threads Broadwell system using sysbench
connect brencmark @40 threads (with select 1 disabled).

10.2 shows moderate improvement: 136219.93 -> 137766.31 CPS.
10.3 is improvement is somewhat better: 93018.29 -> 101379.77 CPS.

Also backported MyRocks memory leak fix from 10.4, which turned out to
be unrelated.