tracing: Remove unused buffer_page field from trace_array_cpu structure

The trace_array_cpu had a "buffer_page" field that was originally going to
be used as a backup page for the ring buffer. But the ring buffer has its
own way of reusing pages and this field was never used.

Remove it.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20250505212236.738849456@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -184,7 +184,6 @@ struct trace_array;
  */
 struct trace_array_cpu {
 	local_t			disabled;
-	void			*buffer_page;	/* ring buffer spare */
 
 	unsigned long		entries;
 	unsigned long		saved_latency;