t1450: convert tests to remove worktrees via git-worktree(1)

t1450: convert tests to remove worktrees via git-worktree(1)

Some of our tests in t1450 create worktrees and then corrupt them.
As it is impossible to delete such worktrees via a normal call to `git
worktree remove`, we instead opt to remove them manually by calling
rm(1) instead.

This is ultimately unnecessary though as we can use the `-f` switch to
remove the worktree. Let's convert the tests to do so such that we don't
have to reach into internal implementation details of worktrees.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>