tracing: Remove redundant #else block for BTF args from README

Remove redundant #else block for BTF args from README message.
This is a cleanup, so no change on the message.

Link: https://lore.kernel.org/all/170952364558.229804.17285528811097152410.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>