Makefile: stop including "GIT-VERSION-FILE" in docs

Makefile: stop including "GIT-VERSION-FILE" in docs

We include "GIT-VERSION-FILE" in our docs Makefile, but don't actually
use the "GIT_VERSION" variable that it provides. This is a leftover from
the conversion to make "GIT-VERSION-GEN" generate version information
in-place by substituting placeholders in 4838deab65 (Makefile: refactor
GIT-VERSION-GEN to be reusable, 2024-12-06) and subsequent commits,
where all usages of the variable were removed.

Stop including the file.

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