xdp: Remove unused mem_return_failed event

The change to allow page_pool to handle its own page destruction instead
of relying on XDP removed the trace_mem_return_failed() tracepoint caller,
but did not remove the mem_return_failed trace event. As trace events take
up memory when they are created regardless of if they are used or not,
having this unused event around wastes around 5K of memory.

Remove the unused event.

Link: https://lore.kernel.org/all/20250529130138.544ffec4@gandalf.local.home/

Cc: netdev <netdev@vger.kernel.org>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/20250529160550.1f888b15@gandalf.local.home


Fixes: c3f812ce ("page_pool: do not release pool until inflight == 0.")
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>