sink.ctf.fs: remove unusued _is_variant_field_class_tag_valid()

sink.ctf.fs: remove unusued _is_variant_field_class_tag_valid()

Clang 8.0:
  translate-trace-ir-to-ctf-ir.c:721:6: error: unused function '_is_variant_field_class_tag_valid' [-Werror,-Wunused-function]
  bool _is_variant_field_class_tag_valid(

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I3794701f857a34d672cc7d297fd7bb7eb5ac31ad
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1766
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>