Remove reference to non-existent sip.conf option

Option was removed in commit 7f883ef4

ASTERISK-25927 #close

Change-Id: I92f9b0196d9fc41d1d58354c07340c465ef1fcf8

configs/samples/sip.conf.sample

@@ -1509,7 +1509,6 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
 ;allow=ulaw
 ;allow=alaw
 ;mailbox=1234@default,1233@default ; Subscribe to status of multiple mailboxes
-;registertrying=yes              ; Send a 100 Trying when the device registers.
 
 ;[snom]
 ;type=friend                     ; Friends place calls and receive calls