location-entry: Reset the lock icon when security level is EPHY_SECURITY_LEVEL_NO_SECURITY

We were returning early leaving the existing icon for non HTTPS pages.
We don't need to keep the gicon and resetting it every time a new
security level is set, we can simply use gtk_entry_set_icon_from_icon_name()
with the name we get from ephy_security_level_to_icon_name().