Remove redundant "create_account_url" string from panel_login.xml.

We already have a "create_account_url" string in strings.xml, and in fact --
according to Eli -- the strings.xml values are "better" for non-English
languages. Unfortunately those "better" strings weren't being used. They were
tied to the FirstRun notification, which seems to have been disengaged.
Changed llpanellogin.cpp to look in strings.xml rather than panel_login.xml
when clicking the "Sign Up" link on the login panel.