tracing: Remove second parameter to __assign_rel_str()

The second parameter of __assign_rel_str() is no longer used. It can be removed.

Note, the only real users of rel_string is user events. This code is just
in the sample code for testing purposes.

This makes __assign_rel_str() different than __assign_str() but that's
fine. __assign_str() is used over 700 places and has a larger impact. That
change will come later.

Link: https://lore.kernel.org/linux-trace-kernel/20240223162519.2beb8112@gandalf.local.home

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/trace/stages/stage6_event_callback.h

@@ -91,7 +91,7 @@
 #define __rel_string_len(item, src, len) __rel_dynamic_array(char, item, -1)
 
 #undef __assign_rel_str
-#define __assign_rel_str(dst, src)					\
+#define __assign_rel_str(dst)						\
 	do {								\
 		char *__str__ = __get_rel_str(dst);			\
 		int __len__ = __get_rel_dynamic_array_len(dst) - 1;	\

samples/trace_events/trace-events-sample.h

@@ -574,7 +574,7 @@ TRACE_EVENT(foo_rel_loc,
 	),
 
 	TP_fast_assign(
-		__assign_rel_str(foo, foo);
+		__assign_rel_str(foo);
 		__entry->bar = bar;
 		__assign_rel_bitmask(bitmask, mask,
 			BITS_PER_BYTE * sizeof(unsigned long));