worktree remove: allow it when $GIT_WORK_TREE is already gone

worktree remove: allow it when $GIT_WORK_TREE is already gone

"git worktree remove" basically consists of two things

- delete $GIT_WORK_TREE
- delete $GIT_DIR (which is $SUPER_GIT_DIR/worktrees/something)

If $GIT_WORK_TREE is already gone for some reason, we should be able
to finish the job by deleting $GIT_DIR.

Two notes:

- $GIT_WORK_TREE _can_ be missing if the worktree is locked. In that
  case we must not delete $GIT_DIR because the real $GIT_WORK_TREE may
  be in a usb stick somewhere. This is already handled because we
  check for lock first.

- validate_worktree() is still called because it may do more checks in
  future (and it already does something else, like checking main
  worktree, but that's irrelevant in this case)

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