lib: remove unneeded forward declarations in trace-ir/field-class.h

lib: remove unneeded forward declarations in trace-ir/field-class.h

`struct bt_field` is not used in this file.  `struct bt_field_class` is
not needed, as the struct is defined right there below.

Change-Id: I4f4da1b32fd80eefecc6205febf51e3e0dd5a365
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7318
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7297