repository: delete ignore_env member

repository: delete ignore_env member

This variable was added because the repo_set_gitdir() was created to
cover both submodule and main repos, but these two are initialized a
bit differently so ignore_env == 0 means main repo, while ignore_env
!= 0 is submodules.

Since the difference part (env variables) has been moved out of
repo_set_gitdir(), this function works the same way for both repo
types and ignore_env is not needed anymore.

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