msvc: Fix macro redefinition warnings

msvc: Fix macro redefinition warnings

Commit 4091bfc (MinGW: Add missing file mode bit defines,
28-12-2009) causes the msvc build to issue many additional
(currently 1008) macro redefinition warnings. The warnings
relate to the S_IRUSR, S_IWUSR, S_IXUSR and S_IRWXU macros.

In order to fix the warnings, we simply remove the offending
macro definitions which, for both msvc and MinGW, are not
required.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Sebastian Schuberth <sschuberth@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>