clar: stop including `shellapi.h` unnecessarily

clar: stop including `shellapi.h` unnecessarily

The `shellapi.h` header was included as of
https://github.com/clar-test/clar/commit/136e763211aa, to have
`SHFileOperation()` declared so that it could be called.

However, https://github.com/clar-test/clar/commit/5ce31b69b525 removed
that call, and therefore that `#include <shellapi.h>` is unnecessary.

It is also unwanted in Git because this project uses a subset of Git for
Windows' SDK in its CI builds that (for bandwidth reasons) excludes tons
of header files, including `shellapi.h`.

So let's remove it.

Note: Since the `windows.h` header would include `shellapi.h` anyway, we
also define `WIN32_LEAN_AND_MEAN` to avoid this and similar other
unnecessary includes before including `windows.h`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>