remove various unused set_memory_* related functions and exports v2

From: Christoph Hellwig
Date: Mon Aug 26 2019 - 03:56:05 EST

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