doc: simplify Makefile using .DELETE_ON_ERROR

doc: simplify Makefile using .DELETE_ON_ERROR

Currently GNU make already removes files when catching an interruption
signal, however, in order to deal with other kinds of errors a
workaround is in place to store target output to a temporary file, and
only move it to its right place on success.

By enabling the built-in .DELETE_ON_ERROR we let make do this task, so
we don't have to.

This way the rules can be simplified a lot.

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>