Enhancement: Skip lock creation on forget if --no-lock and --dry-run

Restic used to silently ignore --no-lock option of forget. It now skips
creation of lock file in case of both --dry-run and --no-lock are specified. If
--no-lock option is specified without --dry-run then restic prints a warning
message to stderr.

https://github.com/restic/restic/issues/3464
https://github.com/restic/restic/pull/3623