Clean-up: rename double 'lttng' prefixed functions

Rename `lttng_lttng_ust_ctl_flush_buffer` to `lttng_ust_flush_buffer`
since the double prefix is odd, but needed to work-around a clash
with symbols introduced in liblttng-ust-ctl as part of 2.13.

Two functions stubs are not used are removed:
  - lttng_lttng_ust_ctl_get_mmap_read_offset,
  - lttng_lttng_ust_ctl_get_mmap_base.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9a924f503c7d409ed1a28768d07cbbad9d23f503