ArmPkg/FmpDevicePsaFwuLib: Fix unused-but-set-global warnings

Remove unused static globals mVendorFlags and mOnRuntime, which are
set but never read, to fix Clang 23 -Wunused-but-set-global warnings.
Also update DEBUG labels to match FwuInfo field names.

No functional change.

Signed-off-by: Qihang Gao <gaoqihang@loongson.cn>