ring-buffer: Remove obsolete rb_event_is_commit()

ring-buffer: Remove obsolete rb_event_is_commit()

Commit a389d86f7fd0 ("ring-buffer: Have nested events still record running
time stamp") removed the only uses of rb_event_is_commit() in
rb_update_event() and rb_update_write_stamp().

Hence, since then, make CC=clang W=1 warns:

  kernel/trace/ring_buffer.c:2763:1:
    warning: unused function 'rb_event_is_commit' [-Wunused-function]

Remove this obsolete function.

Link: https://lkml.kernel.org/r/20201117053703.11275-1-lukas.bulwahn@gmail.com
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>