Platform: Clean deprecated tfm_spm_hal_init_isolation_hw()

SPM HAL implementation has covered the old HAL API
tfm_spm_hal_init_isolation_hw(), it is not used now
and can be removed.

Change-Id: I0b63be8a6ff0e8dca4004bb0992664fed053fb24
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>