tracing: Remove hist trigger synth_var_refs

tracing: Remove hist trigger synth_var_refs

All var_refs are now handled uniformly and there's no reason to treat
the synth_refs in a special way now, so remove them and associated
functions.

Link: http://lkml.kernel.org/r/b4d3470526b8f0426dcec125399dad9ad9b8589d.1545161087.git.tom.zanussi@linux.intel.com
Acked-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>