fgraph: Remove coarse PID filtering from graph_entry()

fgraph: Remove coarse PID filtering from graph_entry()

With PID filtering working via ftrace_pids_enabled() and fgraph_pid_func,
the coarse-grained ftrace_trace_task() check in graph_entry() is obsolete.

It was only a fallback for uninitialized op->private (now fixed), and its
removal ensures consistent PID filtering with standard function tracing.

Also remove unused ftrace_trace_task() definition from trace.h.

Cc: <wang.yaxin@zte.com.cn>
Cc: <mhiramat@kernel.org>
Cc: <mark.rutland@arm.com>
Cc: <mathieu.desnoyers@efficios.com>
Cc: <zhang.run@zte.com.cn>
Cc: <yang.yang29@zte.com.cn>
Link: https://patch.msgid.link/20251126173552333XoJZN20143fWbsdTEtWoU@zte.com.cn
Signed-off-by: Shengming Hu <hu.shengming@zte.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>