tracing/ring-buffer: remove unused ring_buffer size

Impact: remove dead code

struct ring_buffer.size is not set after ring_buffer is initialized
or resized. it is always 0.

we can use "buffer->pages * PAGE_SIZE" to get ring_buffer's size

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>