checkout: drop intimate knowledge of newly created worktree

checkout: drop intimate knowledge of newly created worktree

Now that git-worktree no longer relies upon git-checkout for new branch
creation, new worktree HEAD set up, or initial worktree population,
git-checkout no longer needs intimate knowledge that it may be operating
in a newly created worktree. Therefore, drop 'new_worktree_mode' and the
private GIT_CHECKOUT_NEW_WORKTREE environment variable by which
git-worktree communicated to git-checkout that it was being invoked to
manipulate a new worktree.

This reverts the remaining changes to checkout.c by 529fef2 (checkout:
support checking out into a new working directory, 2014-11-30).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>