powerpc/cell: Drop select of MEMORY_HOTPLUG

SPU_FS selects MEMORY_HOTPLUG, which is problematic because
MEMORY_HOTPLUG is user selectable, meaning we can end up with a broken
.config where MEMORY_HOTPLUG is enabled but its dependencies are not,
leading to build breakages.

The select of MEMORY_HOTPLUG for SPU_FS was added back in 2006, in
commit 4da30d15 ("[POWERPC] spufs: fix memory hotplug dependency").

However we reworked the spufs code and removed the dependency on memory
hotplug in 2007 in commit 78bde53e ("[POWERPC] spufs: remove need
for struct page for SPEs").

So drop the select as it's no longer needed and causes problems.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>