kernel/hung_task: unexport sysctl_hung_task_timeout_secs

kernel/hung_task: unexport sysctl_hung_task_timeout_secs

This was added by the bcachefs pull requests despite various
objections, and with bcachefs removed is now unused.

This reverts commit 5c3273ec3c6a ("kernel/hung_task.c: export
sysctl_hung_task_timeout_secs").

Link: https://lkml.kernel.org/r/20251104121920.2430568-1-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>