The PSA-implementing secure binary is not built using Mbed OS build
tools anymore. Instead, the TrustedFirmware-M (TF-M) build system is
used to produce the secure binary. As such, we remove PSA related hooks
from the build system, remove PSA related scripts from tools/test
folder, and also remove the psa-autogen job from travis which was
running the now unecessary and removed generate_partition_code.py.

Remove the ability to generate new PSA binaries in the old manner, where
Mbed OS implements PSA. We don't yet remove any PSA binaries or break
the currently checked-in Mbed-implemented PSA support. PSA targets
integrated in the old manner will continue working at this point.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>
Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>