Revert "MDEV-12445 : Rocksdb does not shutdown worker threads and aborts in memleak check on server shutdown"

This reverts commit 6f1f9114.

because it doesn't do anything now (the server doesn't check
my_disable_leak_check) and it never did anything before
(because without `extern` it simply created a local instance of
my_disable_leak_check, did not affect server's my_disable_leak_check).