function_graph: Remove the unused variable func

Variable func is not effectively used, so delete it.

kernel/trace/trace_functions_graph.c:925:16: warning: variable ‘func’ set but not used.

This happened because the variable "func" which came from "call->func" was
replaced by "ret_func" coming from "graph_ret->func" but "func" wasn't
removed after the replacement.

Link: https://lore.kernel.org/20250307021412.119107-1-jiapeng.chong@linux.alibaba.com


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=19250


Fixes: ff5c9c57 ("ftrace: Add support for function argument to graph tracer")
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>