modules: Remove fff tests

I've update FFF to run tests in the native CI for GitHub, so we no
longer need to run these extra tests. Remove the fff module from
west.yml since the module was only used in the CI and the header
was directly included in the Zephyr tree.

See https://github.com/zephyrproject-rtos/fff/pull/2



Signed-off-by: Yuval Peress <peress@google.com>