Revert "Fetch header too needs by mailing list"

This has unacceptable side-effects (10x more memory use, extremely slow
folder loading if HEAD is not cached, etc), rendering both large folders
and not fully cached folders basically useless. So, rather break the
mailing list support than those cases.

This reverts commit 35cf4a0b31affe46611e934ebb9312fc9fe3411a.
(cherry picked from commit 6784044a9030a5226b0665de4c15ad66fd212d3f)