uio: remove copy_from_iter_flushcache() and copy_mc_to_iter()

These two wrappers are never used.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211215084508.435401-2-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>