msm: Remove CONFIG_MULTIMEDIA_USE_ION

Back when Ion was first added, a Kconfig was added to allow switching
between Ion and PMEM for allocation. These days, PMEM is completely
deprecated so this config isn't useful anymore. Remove the config
option.

Change-Id: Iadd0f1a01efdc6b5c9c3b0634d92f054f36b323b
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>