Remove unneeded creation of the UST domain sub-directory

ust_app_start_trace() always attempts to create its domain
sub-directory after the channel directory has been created. This is
redundant and can be safely removed.

To provide an example, a "domain sub-directory" is:
/home/jgalar/lttng-traces/allo-20190227-115205/ust

A "channel sub-directory" is, in per-uid buffering mode:
/home/jgalar/lttng-traces/allo-20190227-115205/ust/uid/1000/64-bit

or, in per-pid buffering mode:
/home/jgalar/lttng-traces/cloe_fafard-20190227-115609/ust/pid/hello-ust-20186-20190227-115700

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>