ctf: remove ctf_msg_iter_set_emit_stream_{beginning,end}_message functions

ctf: remove ctf_msg_iter_set_emit_stream_{beginning,end}_message functions

These functions are now unnecessary.  Before the previous patch
"src.ctf.fs: add and use medops to iterate on a ds_file_group using the
index", the src.ctf.fs component used ctf_msg_iter in such a way that
the iterator would process complete data stream files one after the
other, and was reset between each file.  When starting a data stream
file, the iterator needed to know whether it was the first data stream
file for a given stream, and therefore if it should emit the stream
beginning message.  Conversely, it needed to know if the data stream
file was the last one for the stream, and therefore if it needed to emit
the stream end message.

With the new ctf_fs_ds_group_medops, the ctf_msg_iter reads entire
streams, even if they are spread over multiple data stream files.  We
therefore don't need to say whether a particular iterator run is the
beginning or end of a stream, it always is.

With lttng-live, the iterator also always does a single run so always
needs to emit the stream messages.

This patch also removes the related fields in ctf_msg_iter, as well as
the related states in the state machine.

The only non-obvious thing is: what to do with the call to
set_current_stream in check_emit_msg_stream_beginning_state.  This is
the moment where the message iterator asks the medium to provide the
bt_stream instance for the given stream class and stream instance id.
For now, I moved it to just before we need it the first time, which is
when sending the stream beginning message.  This is probably temporary
anyway, as it is planned that we will be able to just pass the bt_stream
when creating the ctf_msg_iter.

Change-Id: I275d4631ff11612abb46c73312bf133753ae4971
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>