Revert "rehabilitate 'git index-pack' inside the object store"

Now setup_git_directory_gently behaves sanely even from subdirs of
.git, so simplify index-pack by no longer protecting against that.

This reverts commit a672ea6ac5a1b876bc7adfe6534b16fa2a32c94b
(excluding tests).

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