manifest: remove mcuboot version for TF-M in Zephyr

We remove the tfm-mcuboot module in Zephyr (which hosts the
mcuboot version that is used for TF-M builds in Zephyr). We
will force TF-M builds in Zephyr to  use the MCUboot version
that any Zephyr builds are using. This will allow Zephyr to
maintain a single mcuboot module version in its default
manifest.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>