Don't clear the username from a realtime database when a registration expires.

Don't clear the username from a realtime database when a registration expires.

Non-realtime chan_sip does not clear the username from memory when a registration expiries so realtime probably shouldn't either.

(closes issue #17551)
Reported by: ricardolandim
Patches:
      reg-expiry-username-1.4-fix1.diff uploaded by mnicholson (license 96)
      reg-expiry-username-1.6.2-fix1.diff uploaded by mnicholson (license 96)
      reg-expiry-username-1.8-fix1.diff uploaded by mnicholson (license 96)
      reg-expiry-username-trunk-fix1.diff uploaded by mnicholson (license 96)
Tested by: ricardolandim, mnicholson

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@286756 65c4cc65-6c06-0410-ace0-fbb531ad65f3