subsys/profiling: use a more universal message when perf is running

"Perf is already running" works for `record` but is a little
strange when doing `printbuf`, so remove the "already" to
make it more universal.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>