Revert "setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR"

Revert "setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR"

This reverts d95138e6 (setup: set env $GIT_WORK_TREE when work tree
is set, like $GIT_DIR, 2015-06-26).

It has caused three regression reports so far.

  http://article.gmane.org/gmane.comp.version-control.git/281608
  http://article.gmane.org/gmane.comp.version-control.git/281979
  http://article.gmane.org/gmane.comp.version-control.git/282691

All of them are about spawning git subprocesses, where the new
presence of GIT_WORK_TREE either changes command behaviour (git-init
or git-clone), or how repo/worktree is detected (from aliases), with
or without $GIT_DIR.

The original bug will be re-fixed another way.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>