Clean-up: remove unused _trace_abi field from the CTF2 trace class visitor

Clean-up: remove unused _trace_abi field from the CTF2 trace class visitor

This unused field results in a build failure when using clang++ 14.0.6
with Werror.

  ./ctf2-trace-class-visitor.hpp:39:37: error: private field '_trace_abi' is not used [-Werror,-Wunused-private-field]
          const lttng::sessiond::trace::abi& _trace_abi;

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib85ad5c9c5affce123ee6300b8549f884a55df31