lib: remove some unnecessary uses of `GString`

lib: remove some unnecessary uses of `GString`

The use of `GString` is unnecessary for these fields.  In fact, I think
it just makes things more complicated for nothing.  Change to use
`gchar *`, allocated with `g_strdup()` and freed with `g_free()`.

Change-Id: I03c6662e97d9cad6e0395fa1e3b29154604e786e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12523
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>