Revert "use custom function instead of RT::Record->Update"

This reverts commits 22b0995c834a6954c030eaee794277132a36c7b6 and
dbdc867b7650a84a5b56dcab984f645ec776caac.

RT::Authen::ExternalAuth is switching to ->Update as well, which lets us
use ->Update here without the waffling problem described by 22b0995.
Using more core code is better.