Revert "fs: add read_iter and write_iter to several file systems"

This reverts commit 707b11d4fbd4ab9e86e9d08a90cfe25d3afc10cf.
Revert "fs: use read_iter and write_iter rather than aio_read and aio_write"

This reverts commit a3aeeac3f3e0dbe492feb2555ea7834dc8154f8d.
Revert "fs: create file_readable() and file_writable() functions"

This reverts commit cc7e32973705bc3fd6a0985fe0b74254f4533238.