Fix: include: remove unneeded declaration of lttng_session_descriptor_get_session_name

There is a declaration of lttng_session_descriptor_get_session_name in
both session-descriptor.h and session-descriptor-internal.h.  Since this
is a function exposed by the API, the one in -internal.h is not needed,
remove it.

Since the removed declaration had LTTNG_HIDDEN, this has the effect of
making the lttng_session_descriptor_get_session_name symbol of
liblttng-ctl exported / part of the ABI. I think it was a mistake that
it wasn't previously exported.

Change-Id: I79d383f012d161a6df42240c6849b1b3af109def
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>