Makefile: don’t add whatchanged after it has been removed

07572f220a8 (whatchanged: remove when built with WITH_BREAKING_CHANGES,
2025-05-12) set up the removal of git-whatchanged(1) when
`WITH_BREAKING_CHANGES` is active. Part of that work was removing it
from `commands` in `git.c`. But the Makefile still lists it as a
builtin. That leaves it in the limbo of being linked but not being
callable; you get the generic error about not being able to call it as
a *builtin*:

$ git whatchanged
fatal: cannot handle whatchanged as a builtin

instead of the expected:

$ git whatchanged
git: 'whatchanged' is not a git command. See 'git --help'.

Based-on-patch-by: Jeff King <peff@peff.net>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>