app/spdk_dd: remove unnecessary initialization value

dd_show_progress() inits speed_type to an empty string,
but that string value itself is never used - it always
gets overwritten by a value from speed_type_str array.

Found by scan-build.

Signed-off-by: Kamil Godzwon <kamilx.godzwon@intel.com>
Change-Id: I9612fd7f806dc34afecba9e6672488e91f451d85
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/18187


Reviewed-by: Michal Berger <michal.berger@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>