remove call_{read,write}_iter() functions

These have no clear purpose.  This is effectively a revert of commit
bb7462b6 ("vfs: use helpers for calling f_op->{read,write}_iter()").

The patch was created with the help of a coccinelle script.

Fixes: bb7462b6

 ("vfs: use helpers for calling f_op->{read,write}_iter()")
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>