Revert "git-am: add am.threeWay config variable"

This reverts commit d96a275b91bae1800cd43be0651e886e7e042a17.

It used to be possible to apply a patch series with "git am mbox"
and then only after seeing a failure, switch to three-way mode via
"git am -3" (no other options or arguments).  The commit being
reverted broke this workflow.

Signed-off-by: Junio C Hamano <gitster@pobox.com>