Revert "Refactor exim::rt to use the new exim template."

This patch somehow glommed on to a different exim
refactor which I'm not ready to merge yet.

This reverts commit b86a33d166c16b9797c817aa7f10eee4359a4a31.

Change-Id: I0faf4e101572ade8de8f808ba10ddf6c05b160eb