Revert "firmware_loader: use kernel credentials when reading firmware"

This reverts commit 5a735811.

Introduces incompatible behavior in android12-5.10 which broke
partner devices. Discussion this topic is in b/222166126. The
new behavior should be fine for android13 and later kernels.

The patch was merged into android12-5.10 with the latest LTS
update.

Bug: 247895237
Bug: 248989172
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: Id8b617b5c91fc080c3b3cbe7ba55cd231bef5cfd