cortex_m: remove last references to debugport_init()

cortex_m: remove last references to debugport_init()

The function debugport_init() has never existed in OpenOCD code,
but few comments erroneously references it in place of the
existing function ahbap_debugport_init().

Commit 00dbc185ee56 ("arm_adi_v5: Split ahbap_debugport_init")
splits the function ahbap_debugport_init() in dap_dp_init() and
mem_ap_init(), but did not removed all the incorrect comments
about debugport_init(). Few of such comments has been removed in
later patches.

Remove the last comment that references debugport_init().

Change-Id: Ibd1f125475386e5653340fedf706903a0ee15897
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6694
Tested-by: jenkins