trace-cmd: Remove trace-cmd plugin handling routines

Currently there are no trace-cmd related plugins, all of them
are designed to be used with libtraceeevnt. As both libtraceevent
and trace-cmd have logic for managing plugins, the one in trace-cmd
is redundant. Those redundant code is removed and replaced with calls
to libtraceeevnt plugin APIs. When trace-cmd has to load any plugins,
it uses libtraceeevnt to do the job.

Removed trace-cmd functions:
  tracecmd_load_plugins()
  tracecmd_unload_plugins()
  trace_util_load_plugins()
  trace_util_read_plugin_options()
  trace_util_free_options()
  trace_util_print_plugins()
  trace_util_free_plugin_options_list()

A new libtraceevent API is added:
  tep_load_plugins_hook() - the local static
function load_plugins() is exposed as API, as this
functionality is needed be trace-cmd.

Link: http://lore.kernel.org/linux-trace-devel/20190802110101.14759-4-tz.stoyanov@gmail.com

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>