Cleanup: remove duplicated implementation of rculfhash

lttng-tools features a duplicated copy of Userspace RCU rculfhash due to
interaction issues between runas clone() and internal libc mutexes.

Now that the runas implementation has been changed to use fork() and a
worker process, we don't need this work-around anymore. Remove the
duplicated rculfhash to lessen the maintenance burden.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>