zephyr: Update docs in test/drivers
zephyr: Update docs in test/drivers

Update README.md in test/drivers to remove
deprecated target.

BUG=none
TEST=none
BRANCH=none

Signed-off-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Change-Id: Ia268508859cb14473bd7ac0a4001a18938a4661c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3463102
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Commit-Queue: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec