worktree: make high-level pruning re-usable

worktree: make high-level pruning re-usable

The low-level logic for removing a worktree is well encapsulated in
delete_git_dir(). However, high-level details related to pruning a
worktree -- such as dealing with verbosity and dry-run mode -- are not
encapsulated. Factor out this high-level logic into its own function so
it can be re-used as new worktree corruption detectors are added.

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