boot/optee-os: remove host-python-pillow build dependency

host-python-pillow dependency was needed to build the STMicroelecronics
version during its rc versions but is is not needed anymore in the release.
It is then useless to keep this dependency.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>