doc: remove unnecessary rm instances

doc: remove unnecessary rm instances

Commits 50cff52f1a (When generating manpages, delete outdated targets
first., 2007-08-02) and f9286765b2 (Documentation/Makefile: remove
cmd-list.made before redirecting to it., 2007-08-06) created these rm
instances for a very rare corner-case: building as root by mistake.

It's odd to have workarounds here, but nowhere else in the Makefile--
which already fails in this stuation, starting from
Documentation/technical/.

We gain nothing but complexity, so let's remove them.

Comments-by: Jeff King <peff@peff.net>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>