drivers: slimbus: Cleanup unused APIs

Since slimbus does not call __depr_arm_iommu_create_mapping during
probe, it does not need to call __depr_arm_iommu_release_mapping during
remove.

Change-Id: I2b1a2095bc352c9262610d58b23f9c15936b8310
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>