cifs: Remove unused code

Remove a bunch of functions that are no longer used and are commented out
after the conversion to use iterators throughout the I/O path.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org

Link: https://lore.kernel.org/r/164928621823.457102.8777804402615654773.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165211421039.3154751.15199634443157779005.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165348881165.2106726.2993852968344861224.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165364827876.3334034.9331465096417303889.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/166126396915.708021.2010212654244139442.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/166697261080.61150.17513116912567922274.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732033255.3186319.5527423437137895940.stgit@warthog.procyon.org.uk/

 # rfc
Signed-off-by: Steve French <stfrench@microsoft.com>