.sparse/makepkg-git-i686: drop dead 32-bit sparse-checkout definition

This file selects exactly the `/mingw32/` paths needed to build the
32-bit `mingw-w64-git` package out of a partial SDK clone. With the
move to UCRT64 we also drop the entire MINGW32 (i686) flavor; there is
no UCRT64 i686 environment, and the third conversion commit removes
every `mingw-w64-i686-*` package outright. The sparse definition can
no longer materialize anything useful, so retire it rather than leave a
file that points at directories that will never reappear.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho