zephyr: zmake: drop platform_ec_dir property
zephyr: zmake: drop platform_ec_dir property

This property is now unused.

BUG=none
BRANCH=none
TEST=zmake testall

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I5c76a85b85a0a071234f4d2e5a4a8582614f4d6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2702503
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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