iwlwifi: mvm: remove d3_sram debugfs file

This debugfs file is really old, and cannot work properly since
the unified image support. Rather than trying to make it work,
which is difficult now due to multiple images (LMAC/UMAC etc.)
just remove it - we no longer need it since we properly do a FW
coredump even in D3 cases.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>