imapserver: Disable compress=deflate extension

It still blocks on reading partial flushes from clients, preventing progress
and eventually timing out. The flate library needs more changes to make this
work.

Connections from iOS mail sometimes timed out, not always.

The extension is simply not announced, code is still present.

imapclient/cmds.go

@@ -150,10 +150,6 @@ func (c *Conn) AuthenticateSCRAM(method string, h func() hash.Hash, username, pa
 func (c *Conn) CompressDeflate() (untagged []Untagged, result Result, rerr error) {
 	defer c.recover(&rerr)
 
-	if _, ok := c.CapAvailable[CapCompressDeflate]; !ok {
-		c.xerrorf("server does not implement capability %s", CapCompressDeflate)
-	}
-
 	untagged, result, rerr = c.Transactf("compress deflate")
 	c.xcheck(rerr)