ath10k: detach coredump.c from debug.c

Now coredump is totally separate from debug.c and doesn't depend on
CONFIG_ATH10K_DEBUGFS anymore, only on CONFIG_DEV_COREDUMP. Also remove
leftovers from the removed debugfs file support.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>