ion: msm_ion: remove platform driver unregister

There are certain issues reported, triggered by an unbind
of the ion platform driver via sysfs. Fixing those results
in the unbind to succeed without a panic, but causes further
issues when ION APIs are accessed after the device removal.
Though there are a number checks that need to be put to fix
these, it is doable. But there doesn't seem to be a real gain
in adding such additional checks in ION code, when the remove
method can be simply removed. The ion device destroy method
doesn't currently perform anything substantial like freeing
up of buffers. So there isn't any gain in supporting the
remove method.

Change-Id: I37476fd5158fe32f729d25840032c3faed136c58
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>