[Pidgin] #5976: Remove HMAC-MD5 duplication from jabber's auth.c.

[Pidgin]

#5976: Remove HMAC-MD5 duplication from jabber's auth.c.
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 Reporter:  QuLogic  |       Owner:  nwalp
     Type:  patch    |      Status:  new  
 Priority:  minor    |   Milestone:       
Component:  XMPP     |     Version:  2.4.2
 Keywords:           |     Pending:  0    
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 I noticed in #5974 that auth.c has some code to calculate a HMAC-MD5
 digest. Since we now have that code in cipher.c, here's a patch to use
 that instead. It also avoids calling `xmlnode_get_child(query, "crammd5")`
 twice.

 It's in need of testing because I don't use the "Force old (port 5223)
 SSL" option.

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Ticket URL: <http://developer.pidgin.im/ticket/5976>
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