Delete unused local variables in the trace library.

trace/mercury_trace.c:
trace/mercury_trace_cmd_breakpoint.c:
trace/mercury_trace_cmd_browsing.c:
trace/mercury_trace_cmd_dd.c:
trace/mercury_trace_cmd_developer.c:
trace/mercury_trace_declarative.c:
trace/mercury_trace_help.c:
trace/mercury_trace_internal.c:
trace/mercury_trace_spy.c:
trace/mercury_trace_tables.c:
trace/mercury_trace_vars.c:
	As above.