Revert "Use exim4-daemon-heavy on the tools mail relay"

This reverts commit 5956da9e8779cc5023d2bff758b55c179fa3c93c.

Change-Id: I04e75e8bd21a617bbc0e161a32dcbc68b4cb1fcd
Merge "Revert "Use exim4-daemon-heavy on the tools mail relay"" into production