kdb: Get rid of redundant kdb_curr_task()

Commit cf8e8658

 ("arch: Remove Itanium (IA-64) architecture")
removed the only definition of macro _TIF_MCA_INIT, so kdb_curr_task()
is actually the same as curr_task() now and becomes redundant.

Let's remove the definition of kdb_curr_task() and replace remaining
calls with curr_task().

Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240620142132.157518-1-zhengzengkai@huawei.com
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>