builtin/pack-redundant: remove subcommand with breaking changes

builtin/pack-redundant: remove subcommand with breaking changes

The git-pack-redundant(1) subcommand has been castrated to require
the "--i-still-use-this" option to do anything since 4406522b
(pack-redundant: escalate deprecation warning to an error,
2023-03-23), which appeared in Git 2.41 and was announced for
removal with 53a92c9552 (Documentation/BreakingChanges: announce
removal of git-pack-redundant(1), 2024-09-02). Stop compiling the
subcommand in case the `WITH_BREAKING_CHANGES` build flag is set.

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