remove various unused set_memory_* related functions and exports v2

[Christoph Hellwig]

Hi all,

while looking into implementing a DMA memory allocator for PCIe unsnooped
transactions I've started looking at the set_memory_* and related APIs,
and it turns out that many of them are unused.  Fix for that below.

Changes since v2:
 - dropped the already merged arm64 patch
 - fix a subject line to not mention the wrong functions