Update tf-a build script

This change removes the tftf.bin from the output folder as it is not
needed after being packed into the fip binary, and can cause confusion
on which binary to use when running the tests.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Idaa44edc58556089983a2c7ff2a2a2a328a71bc3