ring-buffer: Remove stale comment from ring_buffer_size()

It's been 11 years since the ring_buffer_size() function was updated to
use the nr_pages from the buffer->buffers[cpu] structure instead of using
the buffer->nr_pages that no longer exists.

The comment in the code is more of what a change log should have and is
pretty much useless for development. It's saying how things worked back in
2012 that bares no purpose on today's code. Remove it.

Link: https://lore.kernel.org/linux-trace-kernel/84d3b41a72bd43dbb9d44921ef535c92@AcuMS.aculab.com/
Link: https://lore.kernel.org/linux-trace-kernel/20231220081028.7cd7e8e2@gandalf.local.home

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reported-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/ring_buffer.c

@@ -5122,12 +5122,6 @@ EXPORT_SYMBOL_GPL(ring_buffer_iter_advance);
  */
 unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu)
 {
-	/*
-	 * Earlier, this method returned
-	 *	buffer->subbuf_size * buffer->nr_pages
-	 * Since the nr_pages field is now removed, we have converted this to
-	 * return the per cpu buffer value.
-	 */
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return 0;