Remove ht-cleanup thread

The hashtable cleanup thread was introduced to prevent deadlocks
happening when the `cds_lfht_destroy()` function was called concurrently
with userspace-rcu hashtable resizes.

This was fixed in the userspace-rcu project in commit:
  commit d0ec0ed2fcb5d67a28587dcb778606e64f5b7b83
  Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  Date:   Tue May 30 15:51:45 2017 -0400

      Use workqueue in rculfhash

That commit makes it so that the `cds_lfht_destroy()` function can
safely be called within RCU read-side critical sections.

This commit is included in the 0.10 release of urcu. The LTTng-Tools
project now has a minimum version dependency on urcu 0.11.

Because it's now safe to call `cds_lfht_destroy()` within RCU critical
sections, the need for the hash table cleanup thread disappears. This
commit replaces all uses of `ht_cleanup_push()` by `lttng_ht_destroy()`
and remove all uses and mentions of the ht_cleanup thread.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I163a281b64a6b3eed62c58515932f71f3b52fea6