All MBED_SPM targets have been removed previously, via commits
5cc66282dd ("PSOC6: remove PSA targets") and 115b09aba43b ("psoc6:
Remove FUTURE_SEQUANA and FUTURE_SEQUANA_M0"). Remove all the dead
MBED_SPM code, as no targets use it.
Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>