Revert "compat/clipped-write.c: large write(2) fails on Mac OS X/XNU"

This reverts commit 6c642a87.

The previous commit introduced a size limit on IO chunks on all
platforms.  The compat clipped_write() is not needed anymore.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>