tracing: Remove unused function unregister_tracer()

The function register_tracer() is only used by kernel core code,
that never needs to remove the tracer. As trace_events have become
the main way to add new tracing to the kernel, the need to
unregister a tracer has diminished. Remove the unused function
unregister_tracer(). If a need arises where we need it, then we
can always add it back.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>